Given the oil does most of the cooling, a synthetic makes better sense than a mineral oil as the base fluid is inherently more thermally stable and also more responsive to the anti-oxidant additives in the formulation. In hot weather, long hard uphill runs or slow crawls in traffic will give max thermal stress making a synthetic a sensible choice....most of the time on UK roads, a good mineral will likely be enough.....your engine, your choice! The viscometrics will be similar if it is sold as a 20W-50.
