synthetic engine oil
You could also give Mobil synthetic diesel oil a go, its supposed to be the same stuff BMW and Mercedes use on their extended drain servicing in their own workshops, its the synthetic equivalent of Mobil Delvac which is also an awesome oil, check the specs.
I have used the Delvac in my 98' VT since new and also a 76' F350 on LPG camper about 4.25 tonnes, runs up to QLD every year, under a lot of load and hardly uses a drop.
Cheers
Monty