Howdy, On my 03 GMC Sierra with 163,000 miles I have: At 70,000 miles, dropped the pan, replaced the filter and 5 qts. Mobil 1 ATF. At 106,000 miles, did a 100% fluid exchange with Amsoil ATF. I want the truck to last 300,000 miles plus. I pull a 3,000 Lb. boat quite a bit, haul truck loads of wood & run it moderately hard. Which of these should I do? 1) Drain the pan (it has a plug) and add 5 qts. Amsoil ATF? 2) Drop the pan & replace the filter & 5 qts. Amsoil ATF? 3) Do a 100% fluid exchange with Amsoil ATF? Thanks!