I know the manufacturer recommends synthetic only, but is there any harm in using reg 85W-140?. My local parts store doesn't have synthetic. 2003 F250, 215,000 miles
Thanks

 

when i had my 4.30 gears installed a couple of years ago the guy put in normal 85w-140 gear oil in the rear axle. He said it wont hurt at all, its heavy enough to protect the gears and bearings, it wont hurt MPG like people say it does and i wont heat up any hotter than synthetic. Ive towed my back hoe, dump trailer, bed full of wood, topsoil, pavers since then and my rear has never puked fluid of been to hot. Last summer iinstalled the 2010 aluminum rear diff cover and when i dropped the steel diff cover the 85w-140 came out honey color and clean, no metal shavings or anything

 

Probably wouldn't hurt anything, but I would stick with the synthetic.

If your usual parts store doesn't have any synthetic and isn't willing to order it for you, it's time to find a new parts store. This is 2012, the SD is far from the only vehicle that requires synthetic gear lube. Heck, even Walmart carries synthetic.

 

Main difference between conventional and synthetic is longevity. Synthetics go over 3x more than conventional. If you want facts of when to change then send an oil sample to blackstone-labs.

 

Like said. My local Walmart has the 75W140 called for.

 

The title of this thread sounds dirty.

 

I can't believe your local parts store doesn't stock synthetic fluids. While you can run conventional gear oil, synthetic oil has many more friction modifiers and carries a longer service life. Invest up front or over time is what I'd ask.