if only highway miles are involved, and reasonable trip lengths, a good mineral based oil like Caltex Delo 400 or Mobil Delvac MX will easily go 10,000km.
Short trips where the oil never gets to operating temp (in my experience a trip of at least twenty km is needed to open the oil t/stat) and stop start driving stress an oil like nothing else.
I'm a pedant and use the absolutely best stuff I can get my hands on, but any good diesel oil from one of the major oil companies will protect a standard 300Tdi well.
One thing I'm not sure of is how the LPG may react and possibly increase oil nitration. LPG specific petrol engine oils are available to combat potential nitration and it's adverse effects, but has anyone done any testing on LPG/diesel and oil nitration ?

