Got a 502 that seems to be running well but is losing oil pressure. At start up it has 40 psi. Then after about a minute 20 psi. Idle in gear dropping to nothing at all changed the sender and it got worse. No noises from the motor though.

Any ideas???

What kind of oil and how many hours on the oil?

I believe the oil is 20w 50. There is about a half hour on the oil.

I have same motor and fought same problem all season. However my temp was rising as well....

Did you get it straightened out?

No. Without writing you a book, I changed oil 3x. Tried straight 50 w/no change. Back flushed oil coolers which seemed to temporarily work, then removed cooler and inspected, replaced, still same deal. 340 hours on motor. Other motor, runs perfect. Chased it all year and no fix yet. Pulling motors and repower this winter... Sorry man

Rule of Thumb is 10 psi per 1,000 rpm,is this since You changed the oil?did You try a manual gauge yet?If You have a Fram or AC filter that could be the problem,first I would change the oil filter then screw in a manual gauge and see where Your at,The oil would take more than a minute to heat up and lose viscosity,another reason could be if You are getting fuel in the oil,You can tell by smelling it or if the oil level is rising.

It has a mercruiser hi performance long filter on it. How could you get fuel in the oil?

Yes, i agree, it is not heating up enough to thin out. I am hoping that the gauge is bad.

Cut open the oil filter and check it out. Also try a mechanical oil pressure gauge.

cut oil filter open .... first
Mechanical gauge....second
do you have oil temp gauge? that tells everything, if temps are getting hot its usually not a good thing (bearing failure)

then check sender after mech gauge test good...

I don't agree with 10PSI per 1000 rpm, I like 60-80 when over 3000 rpm, I run 75 running hot, 30-40 at idle hot... depends on clearances but I have had issues with low oil pressures on boat engines that run oil thru coolers and such, also make sure you have a 30lb bypass or none at all, and always run hp series filter with no bypass...

good luck!