[ngoma]

Oily belt will cause it to prematurely deteriorate and fail. You'll want to plan to replace it soon.

Valve cover gasket has a few failure modes:

1. There is a commonly available cork-sided version that is a proven leaker. Difficult to get this type to seal. The all-rubber version is better.
Solution: replace cork gasket with good quality rubber gasket.

2. Sometimes the shouldered studs get partially backed out during a VC gasket replacement. Subsequent tightening of the nuts bottoms them out on the (now raised) stud shoulders before sufficiently compressing the gasket.
Solution: Make sure the studs are bottomed in their holes before installing the VC/gasket.

3. Warped/bent VC or incorrect/uneven fastener torque. Too tight may distort the VC sealing surface.
Solution: Inspect VC sealing surface and repair if necessary. Torque down the nuts evenly, in steps, like when torquing down a cylinder head. One or two fingers on a 1/4" ratchet handle seems about right.