Wrong Gear Oil???? Lower MPG's after change...

I was under the mistaken impression that Toyota had changed my gear and trans oil when they did my frame change, but turns out they did not do it like I had asked. It's long overdue, it's been almost 3 years instead of the recommended 2, so I went to Advance Auto for some gear oil. They didn't have any of the manual recommended 75-90, so I bought 80-90 and put that in to both diffs, transmission and transfer case. The week after I changed it we had an arctic cold snap, it's been in the teens down to 0F for almost a week. My truck usually averages about 290-300 miles before the low light comes on, but after the oil change it only made it to 260 before the light.
Is the 80-90 that much heavier than 75-90 that it would affect my mpg that much?
Should I go to the dealer and get some Toyota-brand oil and change it again?

 

I would add the correct weight specified by the manual.
Im interested in you saying you had a "frame change?" Is this because of rot, and if so, why did they not buy it back?

 

My 01 wasn't part of the "buy back" program, that ended with the 2000's, 01 to 04 is just frame replacements.

 

wow! I had not heard of that.

 

i would swap all out to a synthetic 75w90. if the parts store dont have any...have them order it for you. mobil1 or royal purple. the 80w90 is defnitely thicker, and will show a mpg drop...especially in the wintertime. the 75w90 is a bit thinner, but its good year round.

 

Along with the cold did you get snow (i.e. have you been using 4wd alot)? Best bet is to use the recommended weight...

 

I really doubt that it's because of the difference in viscosity.

Both are 90 at temperature.

 

The 80-90 is a bit heavier, but shouldn't be a big difference.

The arctic cold snap is probably what killed you. Did you air your tires up after it got cold?

Remember, colder air is denser air, which means more wind resistance.

It's normal for MPG to go down alongside temperature. I don't think you have anything to worry about at all. I wouldn't do a thing.

 

I agree, the difference in viscosity between 75W and 80W is minimal and only at 0 degree. A number of other factors come into play. I notice a major loss of mileage when the intake air temeperature is low. The ECM must richen the fuel supplied to maintain the proper air/fuel ratio. Also as mentioned , you must keep tires up to pressure. Also if the driving is in wet weather, it causes a lot more drag. Its also harder to push a box shaped truck throught the denser cold air. Clearly snow and 4wd use would lower mileage.

If the driving is not short trips, then the viscosity difference would be nearly immeasureable.

 

Are you talking about gas mileage difference on one fill?

I have the same truck and I also had a frame change this month.
Better ride and no difference in gas mileage.

I had the fluids changed in the f/r differentials, tranny and transfer case. All w/Toyota fluids.
I see they fumbled the fluid changes for you. Sorry....
I had a list I gave to my mechanic to change/replace if he thought needed. Fuel filter and hoses were the only extra parts replaced. Hoses were good but what the heck, one end was already off.

Check your mileage after a couple more fills. They always are going to be different as you'll never drive the exact same roads between fills.

My owners manual (same truck) talks about a low gas gauge but I've never seen it come on, even when sitting on "E".
Does it light the gas pump icon?
Thought about checking bulbs but that seems like a lot of work. Tight spot to check and don't want to screw up anything else as everything works now.

That frame replacement was great huh?
Rides like a new truck!
Also had the clutch replaced while the engine/tranny was on the floor and easy to get at. Saved $300 on labor. The parts were discounted to me also.

Also, once the oil gets warm, it'll still be 90 weight.

 

Yeah, I do love looking under there and seeing nice brand new black paint everywhere, plus the brake cables nice and new. I was a little dissappointed in my dealers service, but overall glad I got it done. I think I'll just leave the oil as it is for now, even if that is the cause of my low MPG's, I figure I'm down maybe 2-3 gallons compared with my former average, which works out to about $6-7 per week extra. If I change the oil it'll be at least $50 worth of oil. Other than possibly causing a drop in mpg's, a little thicker oil won't hurt things, right?

 

That small a difference in oil weight won't be noticeable.
75-90
80-90

The 75 & 80 weight figure comes into play when cold.
Once the oil heats up, (shouldn't take long) you're at the 90 weight. No matter what you put in.