Hi guys,

I managed to convince my friend and his dad to do their own oil changes. They have a 2001 BMW X5 3.0i, and the "correct" oil that was recommended by BMW is Mobil 1 0W-40 (which is what they put in). They've been doing their oil changes at the dealership up until this point, and weren't sure what kind of oil was being put into their car.

My friend's dad is a bit hard to please, so after the oil change he said he didn't like the way the car ran, and thought that the MPG he was getting with the car decreased. What should I tell him?

This is what happens when you try and help someone. Tell him that he requires a level of service that only a dealer can provide, and next change to go back to the dealer.

The official BMW recommendation for that should be something that meets the BMW LL-01 specificiation. The three most common and widely available oils in the US are:

• Mobil 1 0w-40
• Castrol Syntec 0w-30
• BMW branded (from dealer) 5w-30

http://www.bmwusa.com/Standard/Content/Owner/SyntheticEngineOils.aspx

Before this turns into a flame fest, I do have to say that my friend's dad helped me a lot with my car troubles, so try to be nice. :)

And basically, what I want to know is if the performance change is psychological or if there could be a reason behind it... Like let's say going from Castrol Syntec 0W-30 to Mobil 1 0W-40. The viscosity rating is different, so how would that affect the car?

Syntec 0w30 is a thick 30wt and M1 0w40 is a thin 40wt so they're not as far apart as you might think. Don't know what the viscosity is on the BMW 5w30 though.

Even if your friend's dad gets slightly worse mileage he'll still save money doing his own changes rather than going to the dealer. He can always buy the BMW 5w30 from the dealer and then change it himself.