I'd check with Beaumonts but suspect they will say ATF. A decent ATF isnt expensive. In my opinion you are best off using it and upping the service intervals as a change is quick, easy and cheap. Leaving it forever, as people tend to do with transmission oils, does the damage.
If ever needed, Bristol Transmissions built me a stumpy R380 for under £1000 and happily took an LT77 in exchange. Its been spot on.
@vulcan bomber has sleeved one of my cases. The other, wasnt so bad but was still leaking - when it was off for another gearbox problem, I knocked the intermediate shaft out a little, replaced the o ring on the shaft and gave it a good coating of sealant, knocked back in and torqued up. It hasnt leaked since.
I agree in that I have had more.issues with sliding calipers, but interestingly, the issue i seem to frequently have is seized pistons on sliding caliper designs. I have rarely (never actually) had issues with the sliding pins!
I have a 2007. I cannot beleive how bad they are for rust given the originlal purchase price etc. You would think the components should have been much better quality. I'd love to move mine on but the arches need attention before i can do that now.