I've been reading some old posts about syn/dino oils and viscosity, and I am still undecided about my next oil change. My manual basically says use 10w30 cf rated oil. This year I went with Mobile 1 10w30 for my oil change, but I sorta feel like I should be using an oil with a higher rating, like CI 4, just because I want to think I'm protecting my motor better. I thought 5w40 was the latest and greatest weight, but now I don't know. I live too far from the Kubota dealer, so I will probably just hit pep boys or the local parts store. Isn't 5w40 a better all around choice than 15w40 for a once a year oil change( 4 season PA weather)? For that matter, why not use it even in hot climates? Is the thinness at the low end ever a problem? My manual also lists straight weights or 20 or 30 but I'm not going to go there. We always used 15w40 in heavy duty trucks but their manuals specifically called for it. I like to go with what the manual calls for but I'm not sure I like buying old technology.