Changed the oil today at work, the usual 6,000 mile change (even though this change was a bit early) but changed oil from carlube triple r to Mobil 1 ESP 5w-30.

Picked the daughter up, filled up with diesel and tripped the on board computer etc after usual after fill up and made my 16 mile trip way home.

Driving like miss daisy (daughter in back) all the way home, checked mpg as I got out side house and it averaged 69,4mpg!! Usually averages 52 ish??

Well happy with that obviously bit I do the same sort of trip a few times a week and never get those figures.

Have you reset and done same trip before ?

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Yea, loads of times.

Baffled me it has.

Why is everyone using 5w30 now?

In theory a good oil could get you more MPG, a good quality oil will make everything smoother in your engine meaning the whole "conversion of fuel to motion" process is more efficient and you get more energy out for everything you put in. 10+mpg seems a bit insane though, I'd have thought realistically it'd only make 1 or 2mpg difference.

Why is everyone using 5w30 now?

My excuse is that the Mercedes specialist in my units at work gets loads of the stuff, so I buy it off them.

Most cars seem to run it these days.

In answer to your original question - You're dreaming. I'd suggest you might get 1% or so but clearly your "test" isn't objective and just a circumstance of happening.

Edited 16 October, 201114 yr by DRJ

Any difference will possibly be down to the oil being new I'd have thought

Yes, good oils can effect MPG by up to 5-7%. Plenty of research done on it at the Uni where I work. Mostly sponsored by fleet operators who cover 1,000,000's of miles a year with trucks, vans etc. Even 3% can have a massive saving on fuel for them. Most of us wont notice it though. I think the most improvement they got was up to 10%. Couldn't tell me what it was though as it is unbranded marked 'oil 1234#56Ab' etc for blind testing.