I had the dealership change the oil for me 2 weeks ago. (I know, I know, dumb thing to do, but I like to get it done there every once in a while). Anyway, I noticed the invoice says they used "15w40". Do you think It's worth the trouble of taking it back for the proper grade? I'm not sure why they wouldn't know what type VW recommends. Will 15w40 harm anything?