http://www.oople.com/forums/showthread.php?t=110670
I don't know what these K1 3-hole pistons are like, but for my 2WD buggy I use the CORE oils - 500 on the front, 400 on the rear - which works pretty well allround - clay, astro and carpet.
I wouldn't be too surprised if 350 and 400 too soft for astro and carpet though, especially considering the extra weight a 4WD carries - for clay it might work though.