I doubt very much that the tank is leaking. Unless the tank was unused, went through Katrina and had the battery acid leaking on it for awhile.
There is a crankcase vent in back of the motor cases that should be vented to the tank via rubber hose otherwise......

Also, your front sprocket chain oiler could be open, that will put a lot of oil in that area.

Do you have the Amen electric box that sits under the tank? If you do, it will sheild a lot of oil, but some will still get in there.
"It doesn't matter who votes. What matters is who counts the votes." -- Joseph Stalin -- and those good folks in Florida.