I'm interested in removing the valve covers off of our 1997 3.1L Lumina & 1999 Silverado 4.8L to check the internals of the engine (clean, dirty, varnish, etc.). Any advice, and is it very difficult to do?
I wouldn't touch the Lumina. 8 things are liable to fall apart and break and might end up costing you $3890 in repairs during Year 2006.
The truck should only require the coil packs as mentioned. You should have two new gaskets ready and a $20 service manual might be handy also.
Do an oil change in both. Substitute a quart of Marvel Mystery oil in both and run them for 1500 miles.
I assure you it won't hurt either. Especially not that Lumina.
I have a 1995 3.1L Lumina. A quick look through my manual shows that you will have to remove/re-install... the air cleaner assembly, spark plug wires, spark plug wire clamps from the coolant tube, PCV tube, a bunch of little junk if your car has Secondary Air Injection (whatever that means), drain the coolant, and disconnect the coolant bypass pipe to get to the front cover.
To get to the rear valve cover, you also have to remove the ignition coil assembly (which includes the solenoids for the EVAP canister purge control), detach the brake booster vacuum hose, remove the serpentine belt, and remove the alternator, along with it's support bracket. Then, of course, there are the gaskets, and the scraping, and the sealant and bolts and whatnot.
Seems like a lot of work to me just to fulfill your curiosity. Then again, some people really get into this stuff. I don't have a scanner, or I would just upload the whole thing, pictures and all, sorry. Anyway, I hope this gives you an idea of what you would be getting into for your car.