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Trying to decide which weight gear oil to use. I see both 75W90 and 75W140 recommended. Any suggestions on one over the other?

Joe
 

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75w90 is what you should use, 75w140 is to heavy of an oil. 1400hp pro stock cars run 75w90 i think you'll be ok. unless your driving a semi truck or dragracing an 8,000hp top fuel dragster they use 75w140 some 80w140. think of it this way: with 75w140 your trying to stuff pudding down a pin hole.....75w90 is way more thinner
 

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How many quarts of gear oil does the IRS take?
If empty, it will take a little over 2 bottles. If you have a trac-loc rear, don't forget to add the trac-loc addivitive. Ford dealers carry little bottles of it and you only need one. IMO, don't use the bottles at the parts stores with the addivitve already in them.
 

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gear oil

All the truck diff manufacturers spec 75w90. Ford put 75w140 in mine. when I change mine I will go 75w90 synth with the friction modifier. If it can handle 2050 ft/lbs from an ISX cummins, I think it can handle my 4.6 torque.
 
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