The manufacturers always mention maintenance in their manuals included with each locomotive...at least, in my selection they have. Some, like BLI's steamers, they say their running gear does not come lubed from the factory...only their motors and drive train. You should break in BLI engines for about an hour before you lube the rods and valve gear. I always lube the wheel bushings at the same time. I use Dextron III auto transmission fluid for all exterior lubes.
Diesels, I only lube their wheels every 10 or so hours of operation. For steamers and diesels alike, that 10 hours takes me about two years to accumulate for any one engine. You should adjust as necessary.
After the first 10 hours of operation, it would be a good idea to open them up, either type, and clean out the guck from the drive-train. Re-lube with a plastic compatible grease, just enough to coat the surfaces, and call it good for another 10 hours of operating time.
Either type of engine should be inverted after every two hours and inspected with a magnifier and good light. Look for pet hairs, bits of ground foam, lint, arachnid silk and insect bits, grit of all kinds (metal or dirt), and if you can see a fair bit of it wrapped around the axles, bushings, and the wipers, chances are excellent that the inner works could stand some cleaning.
The best maintenance you can give your DCC and sound systems is to keep them isolated from the frame and running as cool as possible. Atop the motor is not ideal, by any means....but you gotta do what you gotta do.
All rolling stock should have the axle tips wiped periodically, their sockets in the side frames cleaned and lubed, and if there are wipers for interior lighting, they must be cleaned as well. Other than that, I dust all items with a light artists brush, not a stiff one, and blow gentely to move the billowing crud away from the model.
Does that help?