Pontius. Suggest you have a friendly chat with your MTO. We've been observing the regs for some years now - the MT chaps were directed to do it. I was enforcing the 'no more than 8 hrs driving in a 12 hr duty day' in 2001 and it wasn't new then.
OC Eng at the time changed his personal practices once we explained the result of fatigue to him (think Selby rail disaster). Whether you're driving an MT car, hire car or your own, you are still doing it for work, therefore on Duty.