MiniD,
Interesting thought by Mr Russell. No wonder it gets very confusing for the average consumer.
Another comment have heard, RACV testing was the source I believe, is that mixing different blends is bad as one additive might react with the other brands additive, so the advice was whatever you use, keep using.
Penrite Classic is something my Father uses in his Side Valvle V8 truck and a Fargo, 46 and 36 respectively. Pentrie recommend HPR30 or 40 for the Mini engine and the capacity says 4.2L(Mini) or 4.3L(Moke) !!! but I just put the whole 5L bottle in and you will get just above the mark, their recommendation always falls below the full mark.
The other deciding factor should be that if you have just paid to have a Mr Russell or other reputable builder put together an engine for your and they say use Brand X, then you use Brand X. If you have issues with the engine and you take it back and say you used Reject Shop 20/50 then you are going to be starting behind the eight ball. So use whatever they recommend for their engines.
Terry