Engine oil, mixing brands/grades?

[Panamax]

There's a big overlap between modern synthetic oils of different grades. In UK the mild climate essentially means the lower figure is irrelevant, whether it's 0w, 5w or 15w.

Significance of the upper figure depends on the type of engine and how it's used. In many cases either a 30 or a 40 should be fine. The 30 spec is often more to do with economy and emissions than engine wear. But you need to be a whole lot more careful if you've got a car designed for 20 where the oilways may be very narrow and thin oil essential. Then there's the question of fancy hydraulic valve timing that may need rapid oil pressure at start-up and correct viscosity to work properly.

The bottom line is use manufacturer's recommended spec oil, or something not too far off. Modern synthetics are incredibly good compared with old mineral stuff that barely crept out of the can/sump on a cold day and got chopped to bits in a few thousand miles of driving.

Back in the day when everyday car oil was heavily advertised on TV there were stories of F1 teams refilling their sponsors' oil cans with synthetic oil from other suppliers.