Oils...
Hi Neil;
As U didn't mention your HGTs Km. I can not say much but, I can advice some in general as a proud owner of a HGT.
Engine; The use of fully synthetic oils improve the protection & engine performance. U can use lots of oils in the recommended range which is 10W-40. Don't use the thinner oils like xW-30, cause they're very thin that can penetrate the combustion chamber unfortunately. Keep the viscosity which is recommended.
I prefer Shell Helix Ultra 5W-40 Fully synthetic !
( In general the first number before the "W" means Winter performance; which must be low, and the last number with two digits means the viscosity. The lower numbers means the thinner oil film between surfaces.)
Transmission; I tried lots of additives & oils brands with lots of failure. Finally I got the best transmission oil by experiencing;
Castrol SAF-XO 75W-90 !!! U can't believe the result.
Hope those stuff helps.
See ya !