I have the oil in my Volvo S60 T5 changed at the dealer since they stock M1. However, since my last oil change not long ago, I am now down 1/2 quart and the oil appears darker than in the past. I am wondering if I really did get synthetic. Is there an easy way to verify this? BTW - I will be changing my own oil again (not just because of this, more because I have had it with this dealer).