I have just took the RR P38 in to change the break fluid, and i asked what one he was going to put in he said DOT 4, i told him i wanted 5.1 or 6 he seemed to be fine with that and wrote it down on the service sheet, got back home and he rang me (I'm talking about a Vauxhall dealership here) and asked why i wanted 5.1 or 6 (i know synthetics are better for a whole host of reasons) and i know there more expensive than mineral ones, i just said I had 5.1 in there already and the car weighs 2.5 ton and it doesn't attract water like the older type fluid, and he seemed happy with that, so as far as i know he's will put 5.1 in, but it just bugs me as to why he would ask anyway??