All good suggestions, and not trying to start a pissing match, but I always overfill mine, as they hold such a small amount. By overfill, I mean I fill till it's running out, then give it another good squirt, and quickly put the plug in. NO LEAKS. The first 2 I had replaced I had not touched, they were as they came from the factory. If the vent is not clogged there should not be any pressure trying to force the oil past that big seal. Heck, you might even be able to do the "seal clean" thing, like can be done to fork seals. Run a thin piece of plastic (coke bottle etc.) around that big black seal, and the metal flange it seals against, I'll bet that would fix it! Look up fork seal leaks