Oil Leak

[76sierraclassic]

I've got an oil leak that I'm wondering if you guys might be able to help me figure out.

I originally thought it might be the rear main since the oil was running down to the bottom of the transmission and dripping off, but I had it in the shop a couple of months ago and the mechanic said he couldn't get any oil to speak of to come out of the rear main. He cleaned everything up real good, but of course it's all coated in oil again. I looked from underneath this evening, and it almost looks like it's coming from the valve covers, but it's not. I don't have a lift or anything, so it was kind of hard to look around, but on the driver's side, the back of the block was "wet" all the way up to the valve cover, and on the passenger side, there was quite a bit of oil residue on the block above the starter.

Does anyone have any suggestions as to where the oil might be coming from?

BTW, this is a stock 350/4bbl with a stock th350 tranny in case you need to know.

Thanks,
Mike