Well here is what the parts guy will tell you.... well one that knows what he is talking about.....
You should use a gasket, get a good felpro one. Use anarobic gasket tack on the cover side to tack the gasket in place, let dry for a min or two. Than bolt her up. If you do it this way you can remove the cover a few times before the gasket is runed. This makes for easy fluid changes if you race alot.
Now on a further note:
When you change your gear set brake them in by driving 20min over 55, let cool for an hour, than drive over 55 for another 20min than let cool for another hour, than you can drive normally for the next 2 or 3 weeks, after than change out the fluild again to remove any metal shavings from brake in, and double check your play in the gears.
I personally prefer Mobile 1 Gear lube.... it is really great stuff. Than I use just a good quality friction mod. with it. And really I would change every 75-100k miles.... but I would also go and replace all the seals at the same time.