I wouldn't worry about draining it soo much... add some octane booster to it, and try to keep it at low revs.... after half tank fill it up with premium and you should be good to go...

btw: milling the heads will raise the comp ratio no matter what... even if he just milled them a couple of thousands.... now the change could be insignificant but it is changed nonetheless

2000 Cougar V6 MTX 2001 3.0L Swap w/ PnP Heads - It ran, it boomed, it RUNS AGAIN Nearly all suspension mods Big ass stereo ...and more... See Her Here