78 C10 oil pressure readings and grounds

[miket007]

I have a 78 C10 big ten 2wd with a GM 350HO, all other parts are factory in readily good condition. Have owned it for 3 years and drive it occasionally. From the day I bought it the oil pressure gauge always read high and would flutter consistently. I considered that to be an issue with the sending unit and the stock factory gauge...no other problems or symptoms. Recently I changed the oil and filter and watched the gauge for a few days...it read a little higher than normal but same behavior....however a few days later I noticed it dropped by roughly half and at idle shows almost zero at 160 degrees when in drive with AC on. No other problems or symptoms have surfaced in the motor which is tight and solid. Now I have read opinions on oil pressure which span left to right....it seems to follow the 10 psi per 1000 rpm. I also understand the sending units can go bad and that proper grounding will wreak havoc on issues like these.

Last night I started troubleshooting the gauge and sending unit first. The truck has a main battery ground running from the battery to the alternator bracket. It then has a ground strap from the pass side cyl head to the firewall. I also see a ground from the battery to the core support. Those seem to be good. I checked resistance of the sending unit (bottom of distributor) from the sending unit to the bolt on the neg ground at the alternator and aside from it fluttering quite abit it reported around 40-45 ohms at idle (750 rpm). When I moved the test probe to the battery neg post it dropped to 20-28 ohms. Now this seems suspect to me but I can't for the life of me figure out why. The cable is good, connections are good and tight... could it be a battery issue and is this possibly the answer to my seemingly weird pressure reading?

Later this week, I will be testing oil pressure at the engine with a real mechanical gauge to rule out an engine problem. Any other thoughts?