There was absolutely no hint that the oil was diluted with petrol. This was an engine that could die well before its full potential life, because the oil was too thin to give good protection. The Castrol recommendation was to check the fuel system, and resample the oil in 2,500 km. The last engine was actually giving lots of hints that it was sick. But it also had abnormal levels of sodium.
Sodium appears in detergents in some oils, and as an antifreeze additive.
And sure enough, every time we checked the radiator level, it was slightly down. This engine also showed quite abnormal levels of aluminium. Something aluminium was wearing very rapidly. But this engine had more problems - the silicon level was abnormally high. This engine had an oil bath filter, and I have always thought that they were difficult to get working properly.