There is supposed to be a slight interference fit between the engine case and oil pump (somewhere around .002 or .003" iIrc- someone please correct me if I'm wrong). The problem these days (and it has been an issue for for I don't know how many years now) is that a lot of pumps are machined slightly undersized, so when the engine warms up the press fit is lost, and now the pump is sucking air as well as oil. Gene Berg Ent. has been o-ringing pump bodies for over 20 years because of this. Some people coat both sides of the pump/case gasket with gasket goop to stop the leakage (with varying degrees of success) but the bearings are still fed aerated oil when the engine is warmed up.
The pump face and/or the cover may not be perfectly flat, which even with the correct gasket (the thinner one is the cover gasket- using the thicker one here will give poor oil pressure at operating temps) could be leaking as well. I think part of the problem is the gaskets themselves these days- they are supposed to be impervious to oil, but like the oil strainer/cover gaskets, a lot of them are cheap uncoated paper and seep oil. The easy solution (without taking the pump out to re-seal it)- wipe the bottom of the pump/case area with a rag before and after going out, and every time you stop when out all day?
Sorry I'm not much more help. Al
PS- If your car is dripping from the oil drain plate after an oil change and you've just put new gaskets in, the oil could be seeping right through the cheap pressed paper gaskets. Tightening the oil strainer nuts is not the answer, as it doesn't take much to dimple the 6mm studs in the drain plate (the torque on those 10 mm nuts is what, 6 or 8 lbs?). As I said above, I've heard of guys putting all kinds of gasket goop on the paper gaskets (one guy on the Samba sprays them on both sides with hairspray before installation; I would hazard a guess that the thinner the sealer is the better.