On trips of over 50 miles or so, once everything gets warmed up, SOMETIMES the oil pressure needle bounces between 50 and 35. This happens when I'm cruising along at 65 for hour after hour. It will fluctuate for awhile and then stabilize for 10 min or so, and then fluctuate again. The oil level reads full on the dipstick after sitting for hours--I never check it right after shutting it down. This occurred both before and after I changed my oil and filter. Any ideas?

 

Check your oil for foaming right after the engine ran.

If there is foam on the dipstick your bolts that attach the suction tube to the oil pump may have come loose.

 

Aha! Thanks, I'll do that. I've always used Rotella, which supposedly is anti-foaming, but who knows anymore!

 

electric gauge???

 

The sending unit IS electrical. Gauge is too I guess.

 

Yes, the oil is anti-foaming, so if you see foam on the dipstick right after running the engine you know that something aerates the oil. If it is on the suction side of the oil pump you can still have pressure but only some oil and a lot of air under pressure going to your bearings.

 

I ran it today at 60-65 for 45 minutes, and the oil pressure gauge started bouncing again. So then I pulled over, shut it off and pulled the dipstick. No foam. That's the good news.

 

Yep, now it's the good question if it's a misindication or if it really bounces the oil pressure.
I'd get an independent mechanical gauge and run it and compare.

 

If the sender is electrical, then the gauge is electrical. You won't have an electrical sending unit initiating a liquid column.

The factory senders are known to be problematic. As Alpine said, put a mechanical guage on it. I had problems with my dash gauge for a long time - I'd get fluctuations like you mentioned, but the mechanical gauge never wavered. I recently pulled the factory sender, and the plastic section had separated from the base that screws into the block.

 

I have had several 12V 2nd gen that have had have on and off oil PSI reading before. I have even had 2 different 2nd gens that the oil PSI would just dive to 0 on and off randomly.

I agree the OEM senders are problematic.

I also agree with installing an extra mechanical oil PSI gauge.

 

Well, with the knowledge that the stock gauge is problematic I would really get some extra gauge and verify if your oil pressure is OK or not.
This will determine what needs to be done.