Noticed even on the 300sd a noticeable performance increase 10/40 vs 20/50.
Mileage increase not more than 1-2 m.p.g.
My 240d averaged 33m.p.g. mixed driving,4 kids,2 adults,luggage.Some in town,Mts.
freeway for 1hr 30min. 80-85 m.p.h. No gear change,stock tires except wider
Not a stock 240d ,number of mods. AS knowledge has increased over the yrs. very sure
could squeeze out a few more. Willing to share how if others want to know.