Problems with 97 Chevy fullsize P/U

[turbosam6]

I went to move my 97 Chevy work truck (Vortec 305, 137,000 miles) and it wouldn't start. The battery is old, so I pushed it up to the garage and threw the charger on it. After maybe 3 minutes on the charger (40 amp), I started the truck, and it fired right up. However, the oil psi gauge was pegged. I mean the highest reading on the gauge is 80 and the needle was buried waaay past that. Otherwise it was running fine. I disconnected the battery and tried again, but still the same. I figured the sending unit was bad, and maybe it shorted and caused the battery to drain. So I let it run, and after a minute or two the gauge dropped to around 30 psi. Normally I see 60 on startup and 20 at hot idle, and since the truck had been running for only a couple minutes it was still cold. The motor sounds fine, no flakes on the dipstick. I drove it and the pressure will rise but only to 35 or so, it did drop to about 18 or so at idle when warmed up. What could be the problem here? My theories:

The battery died and a short in the oil psi sending unit circuit caused it to die and is causing the gauge to read funny.

The motor was locked up and the battery wasn't strong enough to turn it over. Once started it spun a rod bearing, causing the oil psi spike and subsequent loss of psi.

The only other symptoms I have are a light knocking when the motor is driven cold, goes away when warm, I attribute this to the age and mileage of the motor, and I have had to replace the alternator twice. I believe that to be a coincidence though. Also now that the oil gauge reads low, when it does rise its kinda notchy, not smooth like before.

This is my work truck, I really need it! Help me! Thanks!

 

[turbosam6]

Its important to note the light knocking I described has been happening for many miles and isn't new at all. I let the truck warm up a little before I drive it and its fine, oil pressure has been the same for the last 70,000 miles.

 

[KENNE-BILL]

I guess an obvious suggestion would be to hook up a mechanical gauge just to be sure. My '92 S-15 electrical oil pressure gauge pegs at any rpm higher than idle, I trust it like I trust the factory boost gauge on TRs.

 

[turbosam6]

I think the oil pressure sending unit may be bad, but I can't find it. I figured it would be on the back of the intake like all SBC's, but guess not. The hot idle oil psi is still fine, I put a new oil filter on and it didn't help, so screw it, it'll run till it blows.