I have a 99 6.0L Silverado PU with 200K+ miles. I have an interesting problem with low oil pressure at idle on startup. If I keep the engine rpm's up around 1500 or so, pressure seems ok. If I let the engine idle while stopped at a light, the pressure will drop to essentially 0 psi. It has got me a little confused because as the engine warms (oil gets thinner) pressure is fine at ~40psi by the dash gauge at idle. I can understand the oposite behavior of oil pressure dropping as the engine warms because of high (wear) clearances. I haven't checked the sender yet, but I think it is ok as I can hear the lifters starting to click when the pressure goes to 0 if I don't pickup the rpm's until the engine warms. I am thinking the only likely suspect would be the oil pump, but it is hard for me to believe that one might actually wear out. Is there a pressure relief valve that is possibly set too low or broken so it bypasses the passages until the pressure falls (oil gets thinner). I thought I would just throw this out to see if anyone might have had a similar problem, and what the issue was.

Thanks

c3sman