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MG MGB Technical - Oil Pressure Questions

I just replaced my engine with a motor that was rebuilt 20+/- years ago. It was bored 30 over, balanced and blue printed at that time. It had been protected from all extremes and I babied it , turning it over by hand, oiling the pistons etc for weeks prior to actually starting it. Anyway, she runs like a cat on fire BUT the oil pressure "concerns" me at times. At start up it is 70lbs but when I am running her at 70 to 75 mph the pressure can drop to 30. I get no sounds of lack of oil or anything and the performace is not affected. I reduce my speed and the pressure rises and rests around 50lbs at speeds of 65 and below when hot. I am running the 5/30 perscribed by the MGB manual. I am thinking I should try a heavier weight oil?
Peter Murray


I don't recall seeing 5/30 recommended. I thought that for normal ambient temps the recommended oil is 20/50. I believe lighter oils were suggested in colder temps. Double check your source of the oil recommendation.

If a change doesn't fix it, then perhaps the spring-loaded relief valve is not working properly.

Also, if you have an electric oil pressure gauge, you might try it with a mechanical gauge.

C R Huff

5/30 is only recommended for ambient temperatures that are *always* below -10C/15F. For temps that range between +10C/50F to -20C/5F it should be 10W/30 to 10W/50. For temps *always* above -10C/15F it should be 10W/40 to 20W/50.

If you are using 5W/30 at temps above -10C then you *will* get low hot pressures. The Workshop Manual quotes hot running pressures between 50 and 80 psi.

If slowing from 75mph or above to 65mph or below causes an *immediate* increase in oil pressure from 30 to 50 there is probably something wrong in the supply. It *could* be that the pick-up strainer is too close to the pan and can't pickup enough at the the higher revs. Could also be a partially blocked pickup, either with debris or a faulty gasket. Posibly also an air-leak on the suction side. Any of these have the potential to destroy the engine.

However if the pressure only rises *gradually* once you have slowed that is simply viscosity increasing at lower temperatures. As such it is relatively normal, but using too low a grade will almost certainly make it more visible on the gauge. Do you have a oil cooler? Many say they are not needed, but I have read that one of the reasons the factory made them standard was because of complaints of low oil pressure at higher temperatures.
Paul Hunt

I've a mgb roadster 1.8 from 1977.
Can I make use of an actual 15W40 oil?
Here temps range is mostly between +10C


i tried it and it worked well on my 1800 engine, although it seemd to burn a little more than the 20W50.

Change intervals for this oil and the filter are the same as for the 20W50.

I also was told from a garage emplyee that they observed that the discount price quality oils offered at many places tend to bild up more deposits in the crankcase and underneath the rocker cover. So it might be a good idea to avoid the cheap 8 for 5 liters offers and go for a well known brand.


15W/40 is now the 'standard' grade for Castrol GTX, for example, in temperate climates, i.e. never below minus 10C, and 10W/40 should be the minimum. I found no difference in my 4-cylinder after it changed from 15W/50, but got lower hot idle pressures in the V8.
Paul Hunt

Which ever oil you settle on make sure it has some amount of zinc (ZDDP)or add a zinc additive to your oil. The newer oils with no zinc tend to accelerate cam and tappet wear.
John H


I own a MGB Roadster of 1974. Engine has been rebuilt 9 years ago.
Until last month, I used 15W40 oil. But a technical expert advised me to use at least 20W50.

So since november 2008 I'm using 20W60 with an additive (Lucas Heavy Duty Oil Stabilizer)

Oil pressure is now more stable at idle.
- when using 15W40 : 40lbs at idle - 75lbs cruising
- now with 20W60 : 55lbs at idle - 75lbs cruising

So thicker oil is better for the MGB engines since tolerances at that time were larger comparing to modern engines now

DJ Overall

15W/40 is at the lower end of the recommended grades for 'temperate climates' i.e. always above -10C. With that much differnece at idle I'd suspect either a worn pump or worn bearings, albeit still well within tolerance as anything above 10psi should be fine for a hot idle, indeed the Workshop Manual quotes 10 to 25psi. I changed a while ago, on an engine that has done at least 55k, and haven't really noticed any change.

Your additive sounds like one of those snake oils, like STP, which engine and oil manufacturers don't recommend. Thickeners at least can be counter-productive, you may get higher oil pressures on the gauge, but that is because the pump is finding it harder to push the oil through the bearings, i.e. there is less flow. Lucas Oil isn't the same as Lucas electrics.
Paul Hunt


In a large truck engine that was equipped with both an oil pressure gauge and a lube oil temp gauge, I have seen the Lucas (then Morley's) oil stabilizer raise the oil pressure AND drop the oil temp.

It was a reasonably well-controlled experiment because I only stopped for about 20 minutes while crossing the Mojave Dessert. The total quantity in the crankcase remained the same because I drained oil out before adding the Lucas. I can't remember for sure if I added it at 10 or 20% (1 or 2 gallons in this case) because it was back in the mid-eighties.

C R Huff

My understanding has always been that Multiple viscosity oils change character as temp increases. By that, I mean that they start out at whatever the "base viscosity" might be (ex: 10w-30 at cold operating temp is 10w) and as operating temp increases, viscosity index improvers within the oil cause it to behave as tho it were truly a 30w oil.

I live where it gets fairly cold in the winter and when I first start up the GT, my oil pressure is around 40-50 psi even at higher rpms. But by the time the engine is up to full operating temp (195F in my case) the pressure gauge is showing about 70 psi at the same rpms. I use Castrol GTX 20w-50 and given the low ambient temps and the fact that I continue to use this weight oil even in winter, I am not surprised, nor, after 20+ yrs of the same engine, same oil, do I think this is abnormal as the engine warms up.

But I am willing to listen to someone (a petrochemical engineer out there??) who can enlighten me if my observations and knowledge is wrong.
Bob Muenchausen

As John said, ZDDP issue is important for thoses old motors. Have a look in the archives...


Jean G.
Jean Guy Catford

That seems bass-ackwards to me. Oil pressure should be higher when cool than fully hot, as it flows less easily, despite being a multi-grade. This is what I see on the gauge of both my roadster and V8. Whilst multi-grades do give a more even viscosity between 'cold' and 100C than a single-grade to compensate for the very significant thinning of oil as it gets hot, I wouldn't expect a manufacturer to produce an oil that overcompensated for this, i.e. was *thicker* hot than cold.
Paul Hunt

Bob, I have the same problems in cold weather with 20/50 i.e the oil pressure reads about 40psi when the engine is cold and then creeps up to 60psi when the engine warms up. I put this down to the pump cavitating due to being unable to suck the thick oil up quick enough to maintain the higher pressure but I may be wrong.
Iain MacKintosh

Well, Ian, you may have something there. Or maybe I have a clogged oil pickup screen?? I've wondered if the low initial pressure might also have something to do the pressure relief valve in the filter or the oil system. I will have to think about this. All I know at present, Ian is what you and I have experienced.

Actually, Paul, it starts off lower, rises to highest pressure (70 psi) and then falls back off again to about 60 psi after about 15 miles. It does do as you suggest, but I am at a loss for the low initial pressure. It may or may not have anything to do with the oil's viscosity.
Bob Muenchausen


Where in California are you located. With the exception of the high Sierra's California temperatures would not warrant using a 5/30 motor oil. Castrol 20/50 has worked well for me for years.
30 lbs pressure at highway speeds is not good. Depending on the engine builder 60 - 75 lbs is where it should be.

Lee, It's not 30psi it's 40 and the old rule of thumb was that you required 10psi per 1000rpm so it should be fine. It's just a strange phenomenon that lasts for about the first ten minutes or so during which you should never thrash an engine anyway. I,m certainly not worried about it. By the way both my GT and Roadster do this so I think it is more likely to be a characteristic rather than a fault.
Iain MacKintosh

Cavitating or maybe too close to the sump bottom and/or crushed. Many years ago I read an account of an investigation into why oil pressure suddenly and totally failed at high revs. In that case the strainer was very close to the sump bottom simply couldn't pull through the massive volumes the pump shifts at high revs. It's unlikely to be that, unless the pressure drops as the revs are raised (off-load, not the same as thrashing the engine). Odd that your two exhibit the same, as do mine, but different to yours. Smacks of something incestuous ...
Paul Hunt

FWIW, For ZDDP info see my site....
R P Shoebridge

I have always run my B and midget before that on 20W50. I also get lower oil pressures when the engine is very cold. I put this down to the vagaries of the pressure relief valve. Its behavior certainly isn't linear. I can watch the gauge and see when it opens. I have fitted a new valve, new oil pump rotors (flatted the end plate of the housing first) and new big end shells to my original old engine (122,000 miles)
Hot I have between 50 (idle) and 70 psi, its about 65 psi at 2000 rpm. Cold sometimes it seems to stick at 50 psi, but when the revs drop it goes back up to 70. I agree that this could be explained by a blocked strainer, but I have cleaned it and I'm not aware of of the oil pan being dented.
Thats an interesting point about the Zinc, I've always used Duckhams formula Q, but this time I've gone for Texaco Havolene. My engine has suffered from Cam & tappet wear. I've renewed all the valvetrain (except timing gear & chain), but its starting to get noisey again. I'm going to get the followers out over the winter and see what's going on. Can anyone reccomend a zinc additive.
Paul Hollingworth

Some instantaneous variation in oil gauge reading is due to fluttering of the pressure relief valve, which varies with temperature, revs, oil, relief valve, spring and all sorts of things. At some point the factory used a gauge with a sintered plug in an effort to damp out this fluttering but it still happens. I still wouldn't expect to see a *generally* lower pressure cold than hot.
Paul Hunt

Hey Paul, Santa bought me a zenon timing light. Found the distributor advance curves on your site very useful. Its difficult to set 14 deg BTDC @ 600 rpm where the manual says due to the engine not running smoothly. Much easier to set 20 deg BTDC @ 1000 rpm. Found it surprising that my 18GG engine reaches full advance at just 2200 rpm though - Thanks
Paul Hollingworth

FWIW both my Bs show lower pressure on the gauge when cold *whilst running*, but higher *at idle* than when hot. I don't know if it's a clue to what goes on, but on the couple of occasions when I've let the oil drop near minimum on the dipstick, slightly lower than usual cold running oil pressure has been the result, but normal pressure showed once warm.
Steve Postins

I have the same experience with my MGB of 1974 when running in winter days :

start engine when cold : Lbs = 60 at idle
when running with cold engine, Lbs drops to 50 at 2500RPM

however when engine is warm after 30 min, Lbs increases to 70 at 2500RPM and same at idle !

PS : I use 20W60 with Lucas Oil Stabiliser (1 gallon 20W60 oil + 1/4 gallon Oil Stabiliser)


Personally, I think it has more to do with the action of Viscosity index additives of multi-viscosity oils as temperature changes than with either the pump or oil level, altho oil level could be a contributing factor, but for a very different reason.
Bob Muenchausen

This thread was discussed between 18/11/2008 and 05/01/2009

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