To get the valve cover gasket to seal properly, you need to put a small amount of sealer where the flat part of the gasket bends to go over the cam shaft, one small amount on each side of the camshaft, so, 4 total. I'm not sure which sealer is best, I would just go to the parts store and start reading all of the packages of Permatex. The amount of sealer needn't be bigger than about the size of a chocolate chip.

10 years ago or so, I asked the service manager of the local Volvo dealership if I could watch the mechanic do the valve adjustment on my car. He allowed me to watch and it was pretty simple to do, for someone with a lot of experience. I think it took about 15 minutes start to finish. Anyway, that is how they installed the new valve cover gasket, and it didn't leak afterwards.
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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 249K looking forward to 300K badge (or sticker??). >>You haven't really worked on a car until you draw blood<< :-}