I have an 01, 2500 suburban with the 6.0 and have been dealing with a slow oil leak for months.
I first noticed the problem when there were oil spots on the driveway and the check oil level light was coming on every few weeks or so. I ignored the issue at first but then it got annoying putting more oil in it every couple of weeks.
First I wiped everything down and drove it around to try and see where the leak was coming from. There was a lot of oil on the filter and above on the oil cooler. I changed the oil that day and ended up replacing the drain plug (old one had no gasket), replacing the oil cooler gasket (which was damaged), and checking for an old oil filter gasket stuck to the block (which there was not). Problem seemed to be fixed (no more oil on the driveway) but after a month the check oil light came on. After checking, it didn't seem like I lost much oil but the light kept coming on. So, I replaced the oil pressure sensor or switch that's just behind the intake manifold. The oil read at a higher pressure when driving which logically made me think the leak had stopped but it had not.
Last week I wiped everything down again and sure enough there is still oil on the filter and oil cooler (There is also oil around the drain plug, general under area, and back of oil pan but, it is mostly on the filter side).
I don't think (I hope) it's not the rear main seal but I'm not sure.......... Any ideas?