Not quite as simple as that. They were certified for 80 octane LEADED fuel. IIRC, the lead acts as a lubricant on valve guides, etc. 100 "Low Lead" fuel has four times the lead as the old 80 octane, which for many engines, is too much of a good thing.
I run my C85 on unleaded car gas in the summer, but switch to 100LL in the fall due to gas-stability issues. It seems to handle the mixed diet.
Ron Wanttaja