Low oil pressure when hot

15 psi when hot isn't disasterous. What's it idling at and how does the pressure show when you give it some rpm?

Pressure measurement is the resistance to flow, you can increase the pressure with a 'thicker' oil but you're not necessarily getting more oil round the engine.

So I changed to 20w50 oil and it was better for a little while, but as soon as the engine gets properly warm the pressure drops to 5psi no matter what the revs are doing.

Looks like a new pump is needed.

Check the prv, its spring and seat first. But it is probably worn bearings. What's the mileage?

Hi, as said in the second post 'pressure is resistance to flow'. The oil pump produces flow of oil, the narrow gap between the surfaces at the bearings offer resistance. When the bearings wear the gap increases so less resistance = less pressure. With heat comes expansion and bigger gap, heat also alters the viscosity of the oil despite the stabilisers and modifiers in it.

Colin

Thanks for all the replies fellas.

I measured every possible tolerance/clearance between the new and old oil pumps and the old one doesn't seem worn at all!

So I cracked on and changed the Main Bearings and Big Ends (engine and crank still in the car, might I add) so hopefully it'll all be fine when running tomorrow.

The old shells were bad, with scoring and heavy wear. Apparently the oil strainer had blocked with excess silicone a little while ago and it looks like the shells ran dry (not as bad is it could have been though). The crank was still smooth and had no scoring so it looks like I'll get away with it.

I'll update tomorrow, thanks again guys.

Fungus

That will certainly not help the oil pressure. It's a common misconception how oil pressure is maintained in an engine - if the internal tolerances are out by even a small degree the pressure drops because the resistance to flow is decreased. Hopefully that will sort it for you.

That still sounds a little low to me, I wouldn't be surprised if the crank didn't have corresponding wear. Hopefully you have extended the usable life of the engine enough to get a couple more years of use out of it though.