I bet an engine of that size and vintage would like more nitro. Maybe at 15% would be good.
I have a pair of old Irvine 40 ABC engines that are very similar to the old OS 40 FSRs. I ran fuel ranging from 0 to 15% nitro and it performed well in all of them, with some degradation of performance with nitro drop. I have not used that engine in a long time and don't have numbers for you. But my point is that these engines run well with a large range of fuel, unlike some engines that can't handle low nitro or high nitro fuel.