2000 F350 Seems to Eat Starters

I'm on my third start now, duralast golds have a life time warranty so thankfully I'm not out any money but it's still annoying. The first starter I replaced was old, might have even been the original but I can't be sure, I just replaced that one in March, and now in June I've replaced the one from march. I noticed replacing the first one that there was a little bit of oil coming out of the bottom of the bell housing, and now it's even coming from where the starter sits on the bell housing. Could this cause a starter to go out premature? Do duralast starters just suck? When I turn the key it just clicks. Thankfully I haven't been stranded yet but I'd like to get to the bottom of this before I do end up stranded. It also rained heavily both previous times these starters went out? Does rain water typically damage a starter like these?