Jeesh...can oil change the gas mileage that much?

[Sand-Dog]

The truck: 98 XJ, 75K miles, stock 4x4 (including tires), except for Edelbrock header, K&N cold air system, bored-out TB, Dynomax cat and cat-back.

The issue: I bought this relatively low-miles XJ last summer from a neighbor and modified it slightly (as detailed above) to increase the engine's efficiency. With mods done and the PO's original 10W30 oil I averaged 22 mpg on the road (75mph cruise on Interstate).

This weekend, the same trip showed 18 mpg. The only difference is an oil change: went to 20W50 and a quart of Lucas oil conditioner to increase my oil pressure.

Could just the thicker oil have taken the MPG down that much? Other thoughts? Injector replacement time at 75K??

 

[Speed_racer]

Its at 20w when warm, which isnt that much over stock, but to be honest, I cant answer your question.

I will say that if you have an xj that gets 22 mpg, NEVER SELL IT!

 

[98XJSport]

Not sure if you guys get winter gas down there, but that will bring your mpg down a couple miles. Keep in mind there are a lot of factors so to get a true test you should average over several tanks of gas. Other than that, I can't see how oil could really affect your gas mileage that much. But I've been wrong many times before, too.

 

[75SV1]

I noticed a 2 mpg drop in a 351W, when I changed from 10W-40 to 20W-50. So I'd say that you would see a drop in fuel milage. I also notice a 2 mpg increase whan I use synthetics. I don't know what effect the Lucas stuff would have.
Tom

 

[98XJSport]

I noticed a 2 mpg drop in a 351W, when I changed from 10W-40 to 20W-50. So I'd say that you would see a drop in fuel milage. I also notice a 2 mpg increase whan I use synthetics. I don't know what effect the Lucas stuff would have.
Tom

A 2 mpg change could easily be attributed to how pissed you were when you left work, as well.

 

[75SV1]

A 2 mpg change could easily be attributed to how pissed you were when you left work, as well.

Possibly. The change from 10W-40 to 20W-50 was in the late 80's, so memory might play into that. I remember changing back and forth a few times and remember the same results. As far as synthetics, I use my highway trips to PA, a 600 mile drive. Also, if you look at some oil websites, the manufactures want you to use a lighter oil for fuel economy.
Tom

 

[89Daytona]

Using heavier weight oils in winter means you are running thicker stuff through your engine and it has to work harder to push it around which decreases the engines efficiency. 20W50 doesn't flow well compared to 10W30 at low temps.

More oil viscosity stuff:
http://www.micapeak.com/info/oiled.html
http://www.innerauto.com/Automotive_Definitions/Oil_Weights/
http://yarchive.net/car/oil_weights.html
http://www.valvoline.com/carcare/articleviewer.asp?pg=ccr20040601ov&section=hm
http://hypertextbook.com/physics/matter/viscosity/
http://www.bobistheoilguy.com/

 

[Fozzy_Bear]

...
http://www.bobistheoilguy.com/

Bob is "the Man". Definately check out that site.


But to the original question; yes putting that oil in your rig will hurt milage significantly. To be honest, 22 to 18 seems a bit much, but as mentioned, you really have to average a few tanks to get all of the variables to work themselves out. I wouldn't be supprised at all to hear 10% (from 22 to 19.8) is the final number.

But to that end.... Why in the world would you put such a thick oil in an engine with such low milage... In Florida, no less. This isn't a SBC; 75 K on a jeep engine is nothing. - Heck, I've got over 245,000 miles on mine and even I don't use anything that heavy. - What gives?