While I would assume you are talking the same viscosity weight, I would need you to either say it or post it. As one can well imagine there may be things other than viscosity that affect gas mileage.
I had the Castrol OEM 5w-40 (I have to take VW's word for both the brand and viscosity) and ran it as the first 5k mile "break in oil" At 5k miles it was replaced with Mobil Delvac One 5w40 and there was literally no change. I was getting 45 to 51 mpg with the Castrol and continue to get the same with Delvac One.
In another example, I use Mobil One in a Z06 Corvette and over time there is an average of app 1 mpg MORE with 5w30 vs 10w30. (same brand)
Kind of off topic, but the reason I decided not to use the Castrol 5w40 Euro for the Diesel Jetta is the product is available for 7-9 dollars a quart here in the USA and is not very available on the open market. Delvac One 5w40 is also not very available on the open market but not as bad as Castrol AND goes for 5 dollars a qt. Factor in that Castrol is a group III and Delvac is a group IV, and you are getting a superior oil at less of a price. When you can run 10k oil and filter change intervals with Castrol and you can run up to 20,000 oil and filter change intervals with the Delvac One, the BANG for the BUCK becomes even MORE dramatic !
This is per mile lubricated of Castrol: $.0049 cents
(9x4 qts=36, 6.50 oil filter x2=13 36=+13=49/10,000= $.0049)
This is per mile lubricated of Delvac One: $.001325
(5x 4 qts=20. 6.50 oil filter, 20+6.50=26.50/20000= $.001325
73% really gets MY attention!
[ October 02, 2003, 04:06 PM: Message edited by: ruking77 ]