Mineral oil especially formulated for the combined engine and gearbox such as the mini, but for better gearchange etc use MILERS002 except on rebuilt engines where pistons must be allowed to bed in using non synthetic oil. 5 litre bottle.
Semi synthetic has a long life to 100c with maximum working temperature of 130-140c and Millers mineral oil has maximum working temperature of 100c, beyond it starts to oxidise.