There has always been a lot of misconception on oil heat range viscosities tho there is plenty of inter web info to add to confusion. I.E. too much info leads to brain explosion. One point usually overlooked in oil discussion is oil pressure.
Low pressure will result in major problems quicker than wrong viscosity.
I have always felt uneasy in a vehicle that only has an oil pressure light rather than a proper gauge. It won't be long till my new to me td5 has a gauge (or two)