I had a 98 TJ 4 banger and the best fuel economy I got on it was when it was stock. Once the tires, lift, and so on came the fuel economy decreased. If your Wrangler came with the manufacturers book it should have a list of all the fluids and capacities in it. I've always done what is recommended but for oils I use full synthetics. You'll need to remove the diff covers to drain the fluids and you can get a gasket kit or a good seal compound instead.
03 Rubicon a.k.a "RubiX", 35x12.50 MT MTZ, 4" lift and all the other fun stuff Rubicons come stock with such as front/rear lockers, Dana 44s, T-MAX 9000lb winch, and much more to come.
Gone but not forgotten: 98 TJ; 4.5 lift, 33x12.50 Pro Comp MT, and lots more.