I've always used Golden Spectro synthetic 20W-50 with never a failure in over 24 years of really hard use.

I'm looking at running the Motul 5100 (10W40) in the F1 engine, but have been told not to run a part or full synthetic oil until the engine has run in somewhat, instead to use a mineral oil. Does anyone have any opinions on this?

I'm pretty sure Motul makes an engine oil specifically for breaking in a fresh engine. I agree with Richie, I always used golden spectro but it's hard to get so I have been using Motul. Lou

A word of caution, When refitting the belt pulleys be absolutely sure the woodruff key is in place and not pushed back and out when you slide the pulley on. It can ruin your day.
While you have it apart look very closely at the area around the seal and inspect for cracks. If the engine has been run hard enough/long enough it can crack in that area.

motul 5100 for years, too
in laverda 750 and 500 WITHOUT clutch slipping, and in my F1, too