Proper oil pressure behavior?

[aboyandhisdog]

I'm just curious about this and need some knowledge...

On my older engine the cold oil pressure would start out at about 55psi at 2000 rpm's and would drop to 45 psi after the engine and oil were thoroughly warmed up.  That always made sense to me thinking that hot oil would probably not hold pressure as well as cold oil as it would be thicker when cold.

On my new engine, the cold pressure is 35 psi and rises to 45 after the oil is hot.  Exact opposite from what I have become to expect.

Do you guys have an explanation for this?  It leaves me wondering what else might influence a drop or rise in oil pressure.  Inquiring minds need to know!  Thanks a lot!

[SteveKfl]

Was type of oil and the viscosity the same for both engines?

[Rus Curtis]

Same gauge?  After warm up (beyond the odd reversal) does the oil pressure match rpm?

[aboyandhisdog]

10w30 in the new engine, 15w40 in the old.  Both Rotella.

The gauge is the same.  The idle pressure is 20psi and comes up to 45 (hot engine) with most any increase in revs.  I can study this closer to be sure.

These facts aside, isn't it odd that on one the pressure falls when hot and on the other it rises when hot?  I just can't make sense of this...

[oldtime]

Now that's somewhat of an interesting anomaly considering its the same gauge.

The change of oil viscosity could certainly influence the results.
Typically a multi-viscosity oil is made from base petroleum having the same viscosity as it's lowest rating.
Chemical additives are then used to increase the viscosity rating above that of the base stock.

Could possibly be solid debris entered the gauge effecting the reading ?
Was this all noticed using the same engine ?

The fit of various engine bearings will readily effect the oil pressure.
And so does the cam gear jet diameter.
Not to mention the oil pump itself.

[Rus Curtis]

Yeah, weird.  Perhaps a check with a separate pressure gauge.  Verify no debris in the pressure tube (to the gauge). 

Something weird happening right at the start that should have an explanation.

[aboyandhisdog]

Can I assume then that the behavior of the pressure in the OLD engine is correct?  Higher when cold and drops a bit when hot?  What do you guys see?