They are good questions, Bob. The nearest thing to a trick that I know of for getting those grommets to seal onto the tank, is to rotate the round ones, and wiggle the non-round ones a lot after you are convinced you have them on properly, because you never do have them on. One trick the professionals tend to do with some of those items is to lubricate the rubber before fitting it. Friction between grommet and plastic is the main reason it is so reluctant to sit in place properly.
If you are getting leakage even when the grommet is properly put on the tank, then the parts don't fit together. Your tank might have been damaged along the way, or is an unfortunate case of an odd size that is so nearly a standard size that everyone thinks the standard sized part fits it.
I doubt the problem is in the fuel or oil you are using. If that were the problem, the parts would fit together and not leak at first, they just wouldn't last very long.
