Ashcroft:
What oil should I use in my R380 gearbox? We recommend MTF94 in your R380 box.
After a long run in my Discovery, when the gearbox gets hot, the gear lever becomes stiff when moving from side to side. Why? The nylon cage at the end of the selector rail can swell when hot, and cause friction. To rectify this, the parts needs replacing. You can find this part here
It is the same part apparently...
What oil did you put in it?
Lots more FAQs here, decide which best match your symptoms: http://www.ashcroft-transmissions.co.uk/index.php?act=viewProd&productId=218