well I can only answer the air filter oil question. In reality any thick oil will work. I have been using the duratrax air filter oil and it seems to work fine. Thick and goey but I guess that is how it traps dirt.
as for the fuel filter, even if the buggy does come with one already it definately wouldn't hurt anything to have another. You would be amazed how much they catch. If one is good 2 must be better, right?