I have a Volvo 850GLT wagon. In the winter I used Mobil 1 5W30 synthetic. Now I want to use Mobil 1 10W30 synthetic. The garage, do to cost wants to stop using Mobil 1 and recommends German Lubro Moly Voll-Synthese 5W40. Which oil should I use Mobil 1 10W30 or Lubro Moly 5W40? I read somewhere on the net that to big of a range between low and high viscosity rating is no good. Adam