"But I am curioius about that synthetic reference - it's 40 weight? Will that be enough for an XR?"
I read that in the XR750 manual so I assume the engineers at HD think 40 weight was enough? Many of the high end boat motors run on 40 weight and that is about as hard a use as you can give an engine. Full load on a large boat V8 is like pulling a full load on a dyno - or uphill etc. Engines run at those temperatures and loads last a lot longer on oils that don't use VI index improvers and similar non-lubricating additives. The old argument about running thick oil in high rpm engines used to be promoted when using an engine at high rpm as the claim was having some oil stick to the part is better than none. In general thick oils will actually sap a bit of power and cause an engine to run hotter than the thinner oil. Older HD's required thicker oils because of the running temperatures and the clearances they used as a result. If you like to read a lot about this oils, check out the Brad Penn oil web site, or Bob the Oil Guy. Also, the diesel forums go way deep in these areas. I have had very good experience using full synthetic 40 and 50 weight for years in race bikes and street bikes. I have pulled engines apart run on many different oils and it seemed like the Golden Spectro 20/50 Synthetic and Mobil 1 V Twin powered engines always looked clean and showed the least wear of the bores and valves, guides etc. Certainly I would not run a 'car' oil in a motorcycle engine now. They have formulated passenger car oils so they are oriented for economy and the additives they use are not beneficial in an air cooled engine.