The Castrol man said that R747 was what they sold as a mixing oil, in other words it's the point from which one starts to make a special blend according to what you are doing.
At that point you need to know what you are adding, and why. So you might add ash dispersants, detergents, and various anti-wear agents and so on.
He then added that the average kart engine wasn't under that much stress, not like a MotoGP or F1 engine designed to fall apart after 141 laps, not 140 (too frail) or 142 (not close enough to the edge), so that R747 was an adequate oil to race on.
But then we got on to talking about running in an engine, and he saw no reason to use R40 to run in and would have just used 747 but perhaps a bit more per litre; but since running in with R40 worked for us, it wasn't worth the risk of changing.
So, if one is convinced that one gets better performance from mixing the oils, mix them. It doesn't mean the kart goes any faster because you probably aren't running anywhere near the limits where such mixing is beneficial, but you may gain a tenth from the confidence.
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