30 or 15W40 Oil in Dozer?

What weight oil is the best to use in my dozer? Will straight 30 weight oil protect better than 15W40? I only use the dozer when it's warm so I don't really need the lower viscosity. I'd read that today's multi viscosity oils are much better than they used to be so a 15W40 protects as well as 30 and gives you better start up protection.

Opinions are like noses everybody has one. You didn,t say how old your tractor is or anything but if it,s an older tractor & has had the 30 series 3 oil used in it during it,s life then I think I,d stick with that now. I know that most the later engines have been using the 15-40 oil in them & the results are good.These later engines run higher RPMs & closer tolerances in their engineering so I,m sure that,s why the 15-40 is preferred in them.