Millions of R4s over its almost 60 years of existence have done hundreds of millions of kilometers, most of them on single grade mineral oil, and having changed a lot of oil formulation along this time span. I doubt there was a single engine failure caused by inferior oil quality. The lubrication system of this engine is designed around '50s oil technology, and should not need any more than a good mineral oil. There is a good point of argument here, mineral oils of the past were primary products of crude oil, not recycled like today's, but even so, today's low detergent and additive stuff is fine.
There are no weak links in our lubrication system, and points that do not receive hydrostatic lubrication like camshaft/followers or rocker gear are very well conceived and need no further attention, lubricant-wise. Talk about this with a Simca or Ford Pinto Escort owner for example, and you are going to hear horror stories!
Synthetic or synthetic blend oils are a definite no for using in engines without an oil filter, as is the case on pre-70s R4s. The detergent additives in these continuously flush the lubrication system, which is good, but debris, sludge etc. is meant to end in the - efficient - oil filter of the modern engine. Without this, they float around in the oil and usually end up on a main or big end bearing.