I use AMSOIL 5W-30 but I live in New Hampshire where its frickin' cold all the time. With regard to synthetics, there really isn't that much of a difference between 5W-30 and 10W-30. At least that's what my AMSOIL dealer tells me(and he's pretty anal about motor oil). The only difference is the 5W-30 will obviously flow quicker since it is a thinner oil. Quicker flowing motor oil is always better for any engine plus I noticed a very slight improvement in gas mileage. So, if your car isn't burning oil then using a 5W-30 weight synthetic motor oil isn't going to do any harm. I've tried AMSOIL's Series 2000 0W-30 oil weight on my 1996 850R and it didn't do any harm. Of course, if you live in a warm climate I would suggest sticking to 10W-30 or 10W-40. Another outstanding motor oil is Agip 4-Synt. Both AMSOIL and Agip are vastly superior to Mobil 1 but they also cost more. But these motor oils can go 10K miles at least per oil change so if you do 20K miles a year then that's only two oil changes in 12 months.