Completely over thinking this, but I'm good at over thinking! Wife is expecting so we changed her little 2013 Polo for a 2016 Dacia Duster with a Renault/Nissan 1.5DCi engine. My Wife drives cars quite hard however only does short journeys. She drives around 5-10miles a day on average which doesn't give the engine time to heat up properly or give the emissions equipment time to do what it needs. When the baby is here it will be doing more mileage but still not the right mileage for a diesel. I wouldn't have purchased a diesel however we had settled on the Duster and I couldn't find a petrol example with four wheel drive and I don't see the point in a 2WD 'SUV'. I try to ensure once every 2-3 weeks I take it for a run up the motorway to ensure the DPF can regen and to get the oil temperature nice and high to burn off any water. I also only use BP Ultimate or Shell V-Power diesel in it. I'm hoping these two measures are enough to keep it clean and running top for the next 4 or 5 years. The car asks for an ACEA C3 oil in any viscosity. When I picked up the car a few weeks ago I changed the oil and filled it up with Shell Helix Professional 5w30 Dexos2 ACEA C3 and fitted a genuine Renault filter. Here in the UK, ambient temperatures bottom out about -10°c (14°F) with a maximum of around 30°c (86°F). Having had lots of UOA done on my previous cars and seeing significant reductions in wear materials running a 40 weight over a 30 weight I'm keen to use a *w40 of which there's many options. I'm most interested in Castrol Edge 5w40 or Mobil1 0w40 ESP. These oils meet ACEA C3, BMW LL04, VW502/505 among others. I'm sure the above oils will look after the engine perfectly fine for many years to come. However, if I run a 5w30 oil I can have an oil that meets VW504/507 which is a brilliant specification to have. It would probably be either Castrol Edge 5w30 or Mobil1 ESP 5w30. So, if you were in my situation would you run a *w30 and gain 504/507 or loose 504/507 and run the thicker oil?