I have an 04 Mazda 3 liter in which the front valve cover is leaking oil. I know I'm supposed to replace both gaskets at once, but the back one is pretty hard to get to. This van has pretty high miles also. Can I get away with just replacing one, or must I replace both at the same time? Thanks.
You can always change both, one, or none ... it's always your choice. There are repercussions for doing so, though.
With changing both gaskets, you fix the issue of oil leakage. It's a little difficult, but the job is done and you don't have to worry about it anymore. You clean the engine up and you're good to go.
Changing no gaskets means you continue to leak. This is the easiest option, as the only thing you'll have to do is continue to dump oil in it as it will continue to waste oil. Plus, the oil leaks out on the ground (or highway) and continues to pollute the environment and your engine continues to look like a mess.
Changing one gasket means the front of the engine looks better, but the mess of the rear continues. You'll not be leaking as much oil, but you still continue to leak some oil ... it just won't be as noticeable.
For what it's worth, take the time and change out both gaskets. If a job is worth doing, it's worth doing correct. Only changing out one gasket (if both are leaking) is doing a job half-arsed. You'll kick yourself for not taking the extra time and effort to just get the job done. You'll also have the feeling of accomplishment in getting it done, knowing the MPV didn't get the better of you. Plus you'll be doing your part to not waste oil and creating a mess all over the place.