There is cause for concern. It depends on how hard the engine is working in OD. If the person is CLIMBING a grade and is in OD and when they press the gas there is very little response (the engine seems unable to pull the car), then it's probably straining too hard in OD and it should be shut off. If the car is on a level freeway and not climbing, I think leaving in OD is okay.Personally if it were my car, I'd switch OD "off" on any mountain grade or long climb and then turn it back on, on the leve portions of the mountain road.