Hi Ni,
Now you are coming to what can be a frustrating but also rewarding part if owning a performance car
i.e. Knowing what suits your purpose best.
Best option IMO is do some r&d yourself - diff oil isn't too expensive, so try the 90 weight Motul, see what
you think, then try the 140 weight and do the same - that way you will know what is best for YOU
and don't be scared of synthetic oils, synthetics count amongst some of the best in the world, also, not all oils are created equal, some 140 weight oils will give alot better results than others (I have used both valvoline and a
couple of different castrols) I have also used 90 weight, but changed it
out about 2 days later cos I didn't like it
as for roundabouts/tight corners, you will most likely find the diff will lock and the tyrese chirp, or if you are just rolling around, the diff may clunk a little (I have found this is worse with lighter oil than heavier, the heavier oil quirtens down the diff - on the ones I have used anyway)