OK My two cents: Oil does leak by stuff all the time.Multi visc oil allows you to guess what the pressure drop might be and take action regarding the oil pressure relief valve.....Stiffer spring more pressure when hot.But pressure squeezes the oil and causes more heat and some less pressure.So you go to a high output pump If it pumps too much oil two things result.It overrides the relief valve and your back to the temp vs pressure thing again and/or you pump so much oil up to the head on bypass that it gets not only hotter(because of the head's hot) but also overruns the valve stem seals!Soooooo it just isn't simple.
Look at it this way .... If you have a good oil you need no more than 10-15psi for every thousand rpms the motor is running 3000=30-45 lbs
I look at a hot vs cold drop of more than 30% as a sign to start looking.....Lots a things effect this number....An old or worn pump first...next easy is a worn or sticking relief valve.Rocker shafts and bushings and then cam brgs.This order because I've seen a bunch of new cam brgs and not too many new rocker shafts and bushings AND they are easier to change!!!!
If you still have a problem there maybe an internal leak but they are rare.
Main and rod brgs ARE supposed to leak!!!!As the motor turns the little hole in the crank is spraying oil AT PRESSURE!!The oil has to go somewere this is what people call an oil wedge.If you have enough flow the wedge will insure a constant stream of fresh clean oil . If the oil is clean and the volume correct NOTHING TOUCHES.When you get to an oil temp of say 200+ degrees The oil starts to really breakdown and can't maintain (oil has a psi rating at operating temp) it's psi spec and things touch
So if everythings good and you still have a BIG drop CHANGE the oil weight....otherwise you are just masking future problems.Oils have an add pack be sure you match you trucks engine.If you are working it hard .....please pick a good fleet oil ....Rotella or Delo If you have babbit DON'T use a synthetic or TURBO oil You may have nothing to get you home!!
A word on synthetic oils I just don't use em cause in old style motors,like most of ours,They LEAK
on the ground FROM everywhere and they don't help rope seals much.Or oil style rings
Now I'm kindda tired and way too A.D.D. to check this so I hope I got it in some real order that most of you can understand!
I'm sorry this got kindda 'Chiefed' but sometime I just can't 'splain it in a few words
Now if you want it explained further get one a Smokie's books
