I think with hi temps and a heavy van it could potentially be warranted. The life of typical modern synthetic oil will halve for every 10C increase in temp above about 100C. if your typical OC8 synthetic transmission oil is good for 80000km to maybe 120000km with no towing in cool conditions you only need an average temp rise of 20C to get to a 20000 km to maybe 30000 km life. You also only change half the oil for a normal drain and refill change so the average oil life is older than the change interval.
cheers
Rohan
cheers
Rohan