Oil pressure too high?

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Jul 21, 2003
Winchester, UK
Hello everyone,

I have been reading the board for sometime and have been learning a lot of great information. I now have a couple of questions on which you might be able to help me.

I live in the UK and drive a 1976 Bristol 603. This is an obscure British car which uses a 360ci Chrysler engine. The old engine has just been replaced with a new 360ci Mopar Magnum engine (300hp and 375lbft) which now has about 1000m on it. The oil was changed shortly after first start up and again after 200m. It is currently running 20W50 dino until about 2000m.

The idle oil pressure on cold (ambient of say 15oC) start up is very high, i.e. 90psi. When the engine is good and hot the idle oil pressure is about 45psi and the non-idle (>1000rpm) pressure is 80psi. The mechanic who fitted the engine, which is built by Cummins for Chrysler, said they are built with tight clearances and the engine would need a little time to loosen up. It certainly does feel to be getting quicker which more miles on it.


Should I be worried that these high oil pressures might indicate I'm not getting enough oil flow?

If the engine is built with tight tolerances, then would a lower viscosity oil (e.g. 0W30 synthetic) be a good idea when the 20W50 comes out at 2000m so as to get the flow rate up?

Thanks for you help.

PS - I have found a UOA lab in the UK at approx $10 per sample, but have yet to try them.
If the tolerances are tight, then yes, a thinner oil is needed.

I prefer to run a good 5W30 dino or blend for run-in, at least 500 miles, and then drain the machining crud from the engine. I am not sure your engine needs a 20W50 viscosity oil.

A 10W30 or at max, a 10W40 is as high as I would go.
I agree with the 'cule - if you know this engine to be built with tight or even medium tolerances then there is no way it should have or need 20w-50 - ESPECIALLY a dino 20W-50!! I would drain and add 10W-30 if you haven't already.

I have indeed recommended SYNTHETIC 20W-50 in certain cars, in certain situations, but this doesn't sound like such a case.

BTW - COOL CAR!!!!! Pictures?
I agree, 20/50 may be a little heavy for a fresh engine with tight tolerances but you definitely have lots of pressure...a good sign. For now I'm running a 15/40 in my classic Mustang as the Cleveland V-8 engines tend to make heat and I wanted a little extra protection.

I am using the stock pressure gauge which isn't that accurate but I seem to notice an increase in pressure now as the engine warms as compared to the 10/30 that I ran the engine in on. Again I'm using the L/H calibrated original gauge here and have no idea of actual numbers but the 10/30 pressure dropped to almost 1/2 way at speed after the engine was hot.

If your oil pump is maintaining that high an oil pressure with that weight of oil you have plenty of flow I would expect. I too wouldn't go back as far as an 0/30. It wouldn't give you much cushioning on the top of the bearings at start up. I intend to switch to a 10/40 synthetic after a few more miles on petroleum.
All the small-block Chryslers I've had always had high oil pressure from a cold start, but not that high. They would run 70 psi until warmed up, then 50 at cruise and 20 at hot idle in drive. I think the 20w-50 is a bit excessive, 10w-30 would be plenty thick.
MY Dodge Truck had a cold idle oil pressure of 100, warm idle oil pressure was 45 psi. Anything above 1000 rpms hot or cold I was at 100 psi. I would not worry about it. The magnum "LA Series" engines are not a tight engine. They might be tight compared to some of the early british sports cars with tractor engines in them though. That engine will function fine on 20W50 assumeing the weather is not to cold. I have the spec.s for that engine and nothing jumps out as tight!
I know that oil pressure for my Audi 2.8 engine should be as follows:

>25-35 psi at idle

>45-75 psi at 3000 RPM at 175 degr F oil temp
If the oil pressure is lower, then the crankshaft bearings are overly worn.
I agree with JohnBrowning, I don't think a Chrysler 360 V8 could be considered to be a tight motor. I think in fairly warm weather a 15W-40 motor oil would be good alternative to 20W-50. In Winter weather a conventional 10W-40 or synthetic 5W-40 would probably be the best way to go. 5W-40 and 5W-50 would be a good choice all year. 20W-50 should be safe for Summer weather but I don't think it is needed and you would clearly get better performance from a thinner oil. When you have lots of miles and notice higher oil consumption then I would suggest using a thicker oil like a 5W-50, 10W-50, or 15W-50. If you wanted to stay with a 50 wt oil then you could safely use an oil like Mobil 1 5W-50 all year. I believe this oil is available in the UK.

If money is no object then you will want an oil that is ACEA rated A3, B3 and possibly B4. If cost is a concern then ACEA A2, and B2 or API SL or API SL/CI-4 would be the best way to go in my opinion.

[ July 24, 2003, 02:13 AM: Message edited by: Sin City ]
Thanks for the advice.

It looks like the majority view is that I need to back off the viscosity a little. I was worried that the oil was so heavy it was mostly going through the oil pump relief path rather than through the bearings. Perhaps it is just that this engine has a high volume oil pump fitted.

It is atypically warm in the UK at the moment (that's 25-30oC and not raining rather than Texas warm!) so I may stick with the 20W50 for a couple of weeks before it is changed out.

Mobil1 10W30 is not readily available near me. I can get Mobil1 0W40 and German Castrol Syntec 0W30 (called SLX II Longlife I think in the UK). Should I be worried about the 0 rating of these oils at the low temp mark for my engine, which is of an old design. My understanding may be a bit off, but I thought all these oils where of a lower viscosity when hot than when cold and so any engine running viscosity limit would be the 30 hot rating rather than the cold 0 rating. This would then make the question whether a thicker cold rated oil would stay in the bearings etc longer when parked vs the benefit of a thinner cold rated oil giving more flow at startup. I guess I can try it an see what the UOAs say.

My engine was made (or at least assembled) by Cummins and is a gas engine. I think most if not all of the aftermarket Mopar performance engines are built by contractors rather than disturbing Chrysler's main production line for only a few of these engine to be built. I believe this is one reason why my mechanic thought my engine might be built a little tighter than the production line engines - would Cummins build it a little tighter than DC - I would have thought they would rather just build it accurately to spec.

Perhaps I should stop worrying about oil choice and enjoy driving my car in the summertime with its new engine.

Thanks again.

I can get Mobil1 0W40 and German Castrol Syntec 0W30 (called SLX II Longlife I think in the UK).

SLX II Longlife is not the same Castrol 0w30 we've been talking about on here, the one you're looking for is called Formula SLX over there. I believe the SLX II Longlife is not nearly as good.
Definitely reduce viscosity. Pressure is measured by the restriction at the bearings. excess then goes through the pump bypass valve. What is restricted at the bearings has trouble circulating, just like stepping on a garden hose near the start - little comes out the end.
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