New thicker oil = bad gas milage?

Discussion in 'SN95 4.6L Mustang Tech' started by StratGT, Dec 5, 2003.

  1. Hey everyone: I recently switched from Ford 5W-20 to Castrol Syntec 0W-30 and I also switched my K&N filter for the stock filter.

    Here's the problem: My Stang is now making TERRIBLE gas milage. Around 60 miles per tank less. The oil is noticable thicker than the Ford oil (did the OC myself) and I was cleaning the K&N so I installed the original filter.

    Do you think its the air filter? or the oil? Like I said the oil poured thicker than the 0W-30 I installed, however, the engine now runs quieter.

    Any suggestions? :canflag:
  2. Thicker oil results in higher oil pressure and lower mileage which is the primary reason all of the US auto makers switched to thinner oil. Also, with the tight tolerances in mod motors, thick oil is not a good thing. You may not get coverage into some areas causing increased wear and potential mechanical failure. I'd stick with Ford on this one...
  3. Your air filter will also have an impact on things so I think it may be a combination of both items. When you say back to stock filter do you mean a brand new stock filter or the one that had some miles on it before you switched the K & N? Because an old paper filter flows as well as cardboard...
  4. Its very possible the new K&N air filter had too much oil on it and some has been sucked through onto the MAF meter causing a rich condition. Just a thought. I dont see how a slightly thicker weight oil is going to cause a dramatic drop in gas mileage.

    Have you seen a loss in performance or recieved the SES light ?
  5. a 30 weight oil is not going to make a noticiable decrease in gas mileage over a 20w.

    BTW ford switched its oil weight all the way back to 1997 to a 5w-20 in cars that required 5w-30. Just so they could increase their CAFE requirements for gas mileage.