Ed,
There are many factors to consider on this subject. Engine clearances, compression ratio, operational range(rpm), etc... You should consult your engine builder to make sure you have all the data. A couple GENERAL rules: You should avoid
synthetic oil during break-in time, usually at least 5000 miles. Multi-weight oils assume their range by season, meaning 10w-40 or 20w-40 will behave the same in the summer, as they are both 40w at warmer temps. There is lots of detailed information on many company websites, such as RedLine, where you can pick up lots of good information. But, the first step is to understand your engine.
Hope this helps!
~Dave