I use Castrol GTX...

IMO you're just as well off to change the oil at say 3k. I think that you're better off getting the dirty oil out and getting fresh oil in more often. But I know i'll probably get flamed for saying that...
Yea, I know that was somewhat off topic...

Interesting that AMS Oil authorizes that page...

1999 Mystique LS V6 Alpine CDA-9807, Rear deck sub, Opt. TB, K&N RU-3530/MAF adaptor, custom heatshield, FSVT 17's, SCT 4-program chip, Eibach's/ST200's, Hella Fogs...