Flash point looks very good. Unfortunately, they don't specify the test method. I was curious about the flash point since I think it's important in an air cooled motor.
Red Line 20W-50 flash point: 255 degrees C
Mobil 1 15W-50, flash point: 232 degrees C (ASTM D92)
Mobil 1 20W-50, flash point: 214 degrees C ( ASTM D93)
unfortunately, it looks like they used a different test method for the two Mobil 1 products.

https://static.summitracing.com/global/images/instructions/red-synthetic%20motor%20oils.pdf


https://thunderpress.net/editorial/c...2009/02/18.htm

 

Yeah, I'm an ex Amsoil dealer. Here's my take. NO you shouldn't use a syn in an older engine. They were made by older milling machines, ect, ect. The tolerances on gasket surfaces are a little sloppy. AND older engines also have probably been used using older oils that had more wax in them and that made a very good CO GASKET. Then years later the gasket dry's out and all you have is wax sealing the engine. Change to Amsoil and you have leaks. It happened to me on a 1987 Chevy truck I bought used. A buddy of mine had a Shovel head and wanted to run 60 wt in it. He took very good care of the bike and it DID NOT leak oil. I told him no, but he insisted. I wish I had not sold him the oil because on a weekend ride of about 150 miles, I rode behind him. By the time I realized what that **** was on my windshield, I was covered in it. Syn is good in a modern engine. I'm running mineral oil in my M-8 and my twinky because it lessons the top end noise in both bikes. I once rode with a guy who had a Block head that had 200K miles on it ALL with HD mineral oil. It smoked a little on start up, but, still pulled him and his wife around without a problem.

 

So you have highlighted what I wrote earlier that the problem is not the oil but the gasket.

 

When I was young and skint I owned a 1959 BSA C15 'Star', I made my own gaskets for it using a cornflake packet and a hole punch. I set the contact breaker points with my father's Rizla cigarette paper, those were the day's.