Tuning my 98 GS for the Desert

You could hear a pin drop when the almighty "handogg" addressed his adoring fans with these immortal words:

..deserts get cold at night. I wouldn't change your oil weight unless your local Lexus service department recommends it.

..no

...keep it out of the sun..no really. It'll fade the interior. Get tinted windows.

..there's something else going on. If your mpg dropped by 50%, it's not the a/c that's causing it. Modern compressors put very little load on an engine. The mpg difference is about 1 mpg, not 10 or 15.

you're welcome.

Your owner's manual should indicate what type of oil to use, based on anticiapted ambient temperatures. Assuming that the air temperatures drop at night, you may wish to stick to 5W-30.

No, stick to a 50-50 mix.

Keep your tires inflated about 4-5 PSI above the manufacturer recommended pressures listed on the driver's door.

Make sure that the AC is properly charged. Take a look at the sight glass in the receiver drier. The receiver drier is a cylindrical canister mounted in front of the radiator/condenser with a round glass window on top. With the AC compressor engaged, it should look like water flowing past the window. If you see foam, then the AC needs recharging.