What should I do?

I really need your advice on what could be a crucial decision.
I've suspected for some time now that my wife has been cheating on me. The usual signs...phone rings, but if I answer, the caller hangs up. My wife has been going out with the girls a lot recently, although when I ask their names she always says, "Just some friends from work, you don't know them."
I sometimes stay awake to look out for her cab coming home, but she always comes walking up the drive as I hear the sound of a car leaving, around the corner, as if she has gotten out and walked the rest of the way. Why? Maybe she wasn't in a taxi at all?
Last night she went out again and I decided to really check on her. I decided I was going to park my Harley Davidson Lowrider next to the garage and then hide behind it so I could get a good view of the street around the corner when she came home.
It was at that moment, crouching behind my motorcycle that I noticed a small amount of motor oil leaking through the gasket between the rear head and rocker arm cover.
So...is this something I can easily repair myself or do you think I should take it back to the dealer?