i have that same "wet spot" exactly where you are talking about Texas...........i noticed it when i changed my oil 2 weeks ago,
i also use mobile 1, you can't blame the oil, its good stuff.......and i agree with spiller, my car runs better, ie....better gas mileage, quieter, and smoother engine with the mobile 1,
the problme is the gasket, you just need to replace it, parts go bad over time...time to replace it ;)
i won't change the quality of my engine oil for an old leaky gasket (15 bucks),,,,,,i'll change the gasket, (next weeekend)