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The time has come to change the differential fluid (front and rear) in my '06 KJ.
And the immediate question is which fluid to use (which oil grade, syn or mineral, brand)?
I began the research in the forum and found a lot of times the same question. So, there is a lot of
information about the subject and don't pretend to ask the same thing again.
I've read almost every post about what fluid to use in the differentials, not only here but in other forums, including jeepKJ, jeepforum and even a dodge forum.
After all that, one thing is clear, there is not an absolute truth about it. Is more about what worked
for someone or what somebody else think or feels.
I've read about reasons for choosing 75W90 over 80W90 or 75W140. Also read alot regarding which is better for differentials: synthetic or mineral. That's a long, long discussion and I'm sincerely leaning towards thinking that mineral is better, why? because 1st: The main reason Crysler recommends 75W90 (not for towing) to be used on many differentials is MPG, not performance and 2nd, for the front is still recommending 80W90 mineral and the only conclusion I cant get to about why is that 75W90 in front is
not optimal for some reason. (That of course is 100% my opinion and totally argueable).
I know it wouldn't be necesary to question what owner's manual says, but as many of you, what I wanna do is
use only one oil grade for both rear and front differentials.
I have concluded that is best for me to use Mopar 80W90 mineral based on the next statements:
Here in Mexico City, Mopar 75w90 syn is $90.00 MN (aprox. US$7.00) a quart, Mopar 80W90 mineral is $95.00 MN (aprox. US$7.30) a quart, and MOPAR 75W140 syn is $420.00 MN (aprox US$32.30) a quart.
I don't tow anything and don't plan to do in the near future, so because of this and price, 75W140 is discarded.
Winter here is not so severe, it's difficult to see temperatures below 0° Celsius, so it doesn't matter too much if I choose 80W over 75W.
But then, after I've made my mind, came across with a store that sells Mobilube HD 85W140 mineral and is priced $63.00 MN a quart (aprox. US$4.85).
Then, here are finally my questions. Mobil 85W140 is ringing a bell (it's the cheaper one), and knowing that a heavier oil will affect MPG:
If I use Mobil 85W140 front and rear will that be a complete overkill?
Will the MPG increase so much that in the long term I kill the saving in extra fuel (or even get worse)?
Will a 140 grade oil put too much stress in the differential during normal use (not heavy)?
What is your experience with Mobil brand (specifically talking about gear oils)? I know in general is one
of the best and more than cheap is the reason that Mopar is overpriced in my opinion.
I hope somebody finds useful how i came to a conclusion, the only question left is to decide between 80W90 and 85W140.
And, if I finally choose 85W140 based on what you tell here, I'll check the MPG very very close, in order to have concrete measure of how the oil grade affects fuel consumption.
As always, thank you very much in advance for what you might say about this subject.
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2006 Limited 4x4 Silver
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