My daughter has a '03 Volvo V70 with the 2.4 Turbo. The car has 152K miles. We've spent more than the car is worth bringing all the mechanicals up to date, despite doing much of the work ourselves, in the hope it continues to run well. We are switching to Mobil 1 10w40 HM oil from 5w30. Which brings me to my question: Why does the manual recommend 5w30 oil up to 86 degrees F, but says 10w30 is good up to 104 degrees F? I would think the 30-weight end of the 2 multi-viscosity oils would protect the engine to the same temperature. What am I missing here? The viscosity chart also says 5w40 is good from -4 degrees to 104 degrees, which is why we're switching to 10w40 HM. I know the frequent advice on these forums is to follow your owners manual, but sometimes I think they're incomplete or misleading, especially as time goes by. The same Volvo manual says nothing about ever changing the ATF.