Our RAV4 4WD is approaching 60K at which time I want to drain and fill 3-4 quarts of tranny fluid with Toyota WS, and also drain and fill the transfer case and differential. Both of those call for GL-5 80W-90, but I was looking for a good synthetic to extend the drain interval for about 100K. Amsoil lists these recommendations: http://www.amsoil.com/mygarage/ProductRecommendations.aspx?FromIndex=1&url2=2009+TOYOTA+RAV4+B They recommend synthetic 80W-90 gear lube for the differential, but 75W-90 long life synthetic oil for the transfer case. Both list 100K drain intervals. The manual calls for 80W-90 for both...so why can't I use the 80W-90 synthetic gear lube in the transfer case per Amsoil? Is there something in that product that can't be used in transfer cases?