Of course, provided one can get the viscosities one needs. A 15w-40 conventional can meet some very stringent specifications and is very cost effective. It isn't particularly useful when it's -40 outside, though, unless you take other precautions. In the PCMO world, however, more run of the mill viscosities are available in conventional and synthetic (i.e. 5w-30 is readily available in either form, whereas a 0w-40 is another matter).