It does not matter what the first number is when looking at the grade of an oil. I don't remember off the top of my head but the winter grade is checked at 32* and colder as the number goes lower. He was still using a 50 grade oil. I don't know where Gibbs oil is blended at, but if it is on the high side ( so almost a 60) that oil was too thick for that clearance. The spread for any given grade of oil is very wide. Cheap oils run in huge blends can vary in grade from lot to lot. One lot may grade at the top (almost a 60) and the next lot may be on the low side (almost a 40).

There is a reason some oils are $8.00 a quart and some are $24.00. It pays to spend some time researching who these companies are, what base stocks they are using (probably the LEAST important part of any oil...but the company can save $$$$ using a cheaper base stock), what the additive pack is and where they put the oil in reference to grade.

When you start talking about squeezing down clearances and things like that, or using alternative fuels, it pays to do some research. And get on the phone and talk to themes guys. Every oil guy I have ever spoken to has been very frank about the questions I ask.