Not sure how to word this question, but here goes: is there any reason that a single-grade oil can't be as high in various standards as all the usual suspects of x-Wx varieties? I have never had to start my car at a lower temp than say 45-50°F. So I'm thinking a straight grade 30 winter (such as it is) and 40 would work. Is the prime reason for the existence of multi (synth) grades simply to allow for cold starts? I know there are conventional oils, but would like the other advantages of synthetics.