Agree with Phaeton's comments, and I too use Wilko's 20/50W...
Also use it in my mighty Allaggro, all year round, and I have Trialled that, and occasionally driven it very hard!
Dont fall for the later Haynes "Book of Fibs", or even Rovers last/final recommendations of 10/40W or similar, I can assure you that the Gearbox, and especially the Diff Pin will take great exception to that!!
If you run that particular grade, and drive it in "a Sporting manner", it justs wears itself out, twice as fast as normal....