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78 Custom 460 NP435 NP205 Sniper EFI HyperSpark Ignition 4.56 Gears Front/Rear Grizzly Lockers
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4,269 Posts
I would use an 80w90 conventional gear oil of your choice. I'm assuming that was what was used when these were new or something close to it.

I know Eaton does not recommend synthetic gear oils with their Truetrac and Detroit lockers, so I'd imagine if conventional is good enough for them then it's probably good enough for your lock rite.

Link for reference to my Eaton claim for the skeptics. lol.

 

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98 Posts
I would use Valvoline SynPower 75W-140. Interesting the Eaton doesn't "recommend" synthetics for a Detroit locker, honestly I'm way more concerned about lubrication for the gears and bearings than the locker! I'm pretty sure the locker would function with water or mud as a "lubricant".
 

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Ford Hoarder
78 & 92
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8,675 Posts
Sometimes the synthetic can mess with the lockers ability to lock, or the lock up ratio. From my understanding of it.
I personally run amsoil 75-140 in most of my things.

Though I feel that any decent gear oil and you will be fine. More important to service it regularly then what oil you use, unless you are really pushing everything to its limits.
 

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Premium Member
78 Custom 460 NP435 NP205 Sniper EFI HyperSpark Ignition 4.56 Gears Front/Rear Grizzly Lockers
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4,269 Posts
That's essentially what I was told at my local 4x4 shop when I asked about gear oil for my new gears/lockers. The guy said as long as I was changing it that it was less of a concern as long as the oil was of good quality. Gear oil is probably the least changed oil in any DD vehicle.
 

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Moderator and Scrounger Extraordinaire
85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
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19,433 Posts
I run a quart of redline heavy shockproof mixed with lucas HD synthetic 75w90 in all my differentials. Even the detroit AND trutrac. Though they are pre-Eaton units. The heavy shockproof really cushions the gear mesh nicely and has excellent cling properties.
 

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Moderator and Scrounger Extraordinaire
85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
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19,433 Posts
Absolutely! I will be doing so when I change the oil in my NP435. I think next axle oil change in the bronco will go full strength heavy shockproof. 37s and lockers are rough on half ton axles.

Make sure to run a gear lube in all cast iron cased gear box assemblies. Synthetic vs dino juice doesnt really matter except with friction clutch based limited slips. And anything you put in the trans must be "yellow metal safe" so it doesnt damage the synchros. Aluminum and magnesium cases get ATF.
 

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84 Bronco, 351w, c6, custom doubler, np208, 5.13’s, TTB44, 9”, locked f/r
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1,398 Posts
I wouldn’t run heavy shockproof in an np435, I don’t think the synchros would like it. I would just use redline MTL in the trans
 

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Premium Member
78 Custom 460 NP435 NP205 Sniper EFI HyperSpark Ignition 4.56 Gears Front/Rear Grizzly Lockers
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4,269 Posts
That or the MT90. They both say they are safe with brass parts.
 
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