People think it's the sump gasket, but it's actually the oil pump or smaller gaskets around the fuel filter.

Clean and degrease the oil as best you can, put it up on blocks and run it for a bit... If the area near the oil pump starts dripping and weeping, you know that's where it is.

It could also be a topside gasket, like on the crank case, or an intake manifold, but those tend to be more obvious (because you can check them standing up).