I run Amsoil 5w-30 in my 1MZ-FE. and I'm just kinda concerned as to the drain interval. Amsoil is the finest oil I can find, and I would like to do the 6 months filter change and use the oil for a year, but I've done some research and from what I've read with the sludging problems, I'm wondering if I'm pushing my luck? I can get Amsoil for about $6 a quart, so price isn't too bad, do you think I should just change oil when I change filter? So what actually causes sludging of oil? I'm not sure I totally understand sludging oil, is it just getting real thick? Thanks for insight.