[Wavery]

If you are towing down-hill or on flat ground and the trans is not "Searching" for lower gears, you may be OK using OD on most vehicles.

Once you get into rolling hills or even a head-wind that causes down shifting, get out of OD immediately until you are back on flat land. It has nothing to do with the engine size.

As was correctly mentioned previously.........gas is a lot cheaper than transmission work. It's even cheaper than the additional trans services that would be needed due to towing in OD.