On my 1988 Chev C2500 the 305 engine has always had medium to low oil pressure (tired with 145,000 miles) at operating temp. Now the oil pressure drops to zero as the engine temperature rises to normal. I installed a new oil pump, inlet screen, intermittent shaft and pressure sensor/sender. I can drive vehicle with no discernible bearing noise or other engine noises which may be from lack of oil. Since problem developed I've driven only 30 miles at one time so no great distances. What could be wrong? (I installed 5 qt of synthetic oil and that is when problem developed).

 

What weight?

 

5.5 quarts of a Quaker state synthetic; first time I've used a synthetic oil-purchased because of price. I purchased the truck with 115,000 miles and recently installed an intake manifold gasket with other repairs. Always had so-so oil pressure but never dropped to zero until the synthetic oil was installed; coincidence? Truck has always ran terrific and is quite solid for my area in Wi.

 

The oil reads near 30 psi when I start and remains at that pressure for the first few miles on the road, then slowly drops to zero. Gauge reading does not fluctuate with RPM which it used to do in the past.

 

I have always used 10/30W conventional oil. The vehicle does not run daily-6000 mile a year at best.

 

I would switch back to conventional oil and try 20W/50 to see if problem goes away.

 

That would be my suggestion. Synthetic only increases the life of the oil.

 

I'll try that weight, although I have run straight 30W with the same results. If no oil is reaching the rocker arms, lifters, or any other pertinent parts, how long would the truck travel down the highway before I'm hearing valve train noises, bearing noises, etc? I've read a forum somewhere that a vehicle lost oil pressure instantly while on the highway and the motor was destroyed before they could pull onto the shoulder. I also like to add .5 to 1 quart of Lucas oil at service time. Thanks for all the great advise.

 

The oil viscosity heavier 50w will be just that at higher temps. This will be "thicker" (laymens terms).

 

I'll try it. The worst thing to happen by driving her with no pressure is extensive motor damage either way. The only issue is the farther I drive the farther the tow truck will have to haul her back home.