Elf Excellium 5w40 from the dealer (got mine for about £23 for 5 litres) is pretty good I've found.
Alternatively, Silkolene Pro S(?) is a 'proper' synthetic and is supposed to be better but is a little more unless you get about 24 litres bulk buy.. lol
edit: I would have thought that 15w50 was a bit thick for winter myself, but I guess it is getting warmer now. (allegedly! lol)