20w50 because that's what the factory reccommend... and they knew a thing or three about the engines.

Used LUCAS 20/50 high zinc break-in and operating oil on the new 1185cc build.  Formerly 20/50 Castrol in 998 & 1380 and the cheapest 10/30 oil one could fine in Australia for the original 1098.  The latter worked well on a 70-day road trip.

I have seen either 20W/50W or 10W/40W used by most Mini mechanics. I use 10W/40W since it is recommended
by my Mini specialist with 40+ years working only on Minis and Mini Cooper S's. He told me to be sure to add a little bottle of ZDDP additive (zinc) if it is over 30 years old. Mine is a '68 so this was a necessary step as newer cars have cat converters that can't handle the old fashioned oil that came with zinc. Yours will apply to this as well.

I have always  used Straight 40 W.. in all my 550 plus minis/mokes  that I have owned and sold..  BUT most folks do NOT do this..  I do this because the tranny and engine use the SAME oil.. later bc

what weight of oil  Castrol 10w 40,0r 20w 50