The truck we have is a 1999 Chevy Suburban 1500, 2-Wheel Drive with a 350 vortec engine. We replaced the upper and lower intake manifold gaskets with no leaks for about a week and several hundred miles then it started leaking oil and then antifreeze after that. Does anybody know what the problem might be? Thank you for any help in advance.

 

Likely headgasket/ worn or stuck piston rings, or valves stuck.

I have the same issue in my 99 tahoe. No visible leaks for oil and antifreeze.
Did a compression test and cyl 6 is little lower but enough to misfire.

I have been driving it like this for 10k miles.
Im in Europe so parts are scarce here. waiting for engine to blow so I can either part it out or throw a new engine in it....