I don't know what the climate in New Mexico is like. But being a southwestern state, I think that oil weight is a bit light to use. As far as regular vs. synthetic goes, beats me! I just use regular Castrol GTX. Although given the climate I'm in, and my driving habits, I use 20w50. For the proper oil range, check the archives for the reccomendation list. As far as oil additives go, stay away from them! All that you'll need to keep the motor running fine is going to come in the bottle of oil you buy. Slick 50, Prolong, Dura Lube, Z-Max, and others are a waste of time and money. The only additive I've ever used was CSL. And even then, I noticed only a tiny difference in improvement. Small enough that it may have all been in my mind... If you've already got a bottle of Slick 50, I'd just take it back. -Robert vin 6585 "X"