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rear end lube question


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I removed the fill plug to check the level in our 2004 1500 Silverado. thinking it was low so i filled it up with 75-90w. it came out of the hole. Put the plug in. reading up on this topic i found out it might have synthetic lube in it. Question, will it hurt to top it off with regulator lube or should we change it out? I did stick my finger int he hole but didn't get any on my finger thinking now it might be synthetic. Any suggestions? great site  a lot of info here! Tks john

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21 minutes ago, John Coody said:

I removed the fill plug to check the level in our 2004 1500 Silverado. thinking it was low so i filled it up with 75-90w. it came out of the hole. Put the plug in. reading up on this topic i found out it might have synthetic lube in it. Question, will it hurt to top it off with regulator lube or should we change it out? I did stick my finger int he hole but didn't get any on my finger thinking now it might be synthetic. Any suggestions? great site  a lot of info here! Tks john

 

 

Factory fill is synthetic.  

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7 hours ago, John Coody said:

I removed the fill plug to check the level in our 2004 1500 Silverado. thinking it was low so i filled it up with 75-90w. it came out of the hole. Put the plug in. reading up on this topic i found out it might have synthetic lube in it. Question, will it hurt to top it off with regulator lube or should we change it out? I did stick my finger int he hole but didn't get any on my finger thinking now it might be synthetic. Any suggestions? great site  a lot of info here! Tks john

If you are using the fill plug on the side, the fluid is to be 5/8 to 1-5/8 below the opening and Synthetic gear oil has always been called for, especially if she has the G80 locker.  75W90 is the correct weight.

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On 9/11/2023 at 7:30 AM, John Coody said:

I removed the fill plug to check the level in our 2004 1500 Silverado. thinking it was low so i filled it up with 75-90w. it came out of the hole. Put the plug in. reading up on this topic i found out it might have synthetic lube in it. Question, will it hurt to top it off with regulator lube or should we change it out? I did stick my finger int he hole but didn't get any on my finger thinking now it might be synthetic. Any suggestions? great site  a lot of info here! Tks john

 

2004 I see. Likely a high miler? Change it out. If you knew how quickly most gear oils, including synthetics, shear down a grade or more it would frighten you. 

 

16 hours ago, Black02Silverado said:

AMSOIL' s Severe Gear is a good option.  If you are interested let me know and I can quote some delivered.

 

Severe Gear 75w-90

 

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This one will not shear down nor will any of the Red Line gear oils. Even though I'm a Red Line guy I use this one. The EZ pour package makes it a breeze to use. 

 

You can mix synthetic and conventional. Nothing bad happens. Nothing good happens either. There is no synergy in this blend I know of. 

 

If you don't like the expense of the two offered, then 'Grade Up" to 75W110 in something like Lucus or Valvoline, Castrol, you choose. It will shear down to the 90 in a few thousand miles. 

 

IF you have a larger backlash giving you bit of clunk going R to D you may even consider 75 or 80W140. Not a fix for bearing growl but helps with diffs with normal wear near service life end. 

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9 minutes ago, dna9656 said:

I once changed out the lube in an aging Oldsmobile Cutlass, some one had added a thickening agent to the gear lube....saw dust.

 

Barney Fife bought a car with sawdust in the steering box, manual transmission and diff. 

 

 

life lessons andy griffith show

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Kidding aside. Viscosity can be a useful tool. I own a 2005 Harley TC88 five speed. The transmission calls for a 20W50 fill. With that fill it shifts just okay cold and like crap hot. Fill it with Red Line Shockproof Heavy and it becomes almost Japanese in nature. Hot or cold, slick as silk. Yea, it also has to do with the friction package, but it gets rid of the Harley hot CLUNK. I've also used this fluid in an Albion four speed (Royal Enfield) whose original lubricant was 00 grease to absolutely transform its shift quality. 

 

Around 175-200K even the best of diffs will get a bit of click-click caused by?....increased clearance as the gears wear. A slightly thicker more viscous oil cushions that enough to hold off a rebuild awhile longer. 

 

Or use sawdust :crackup:🥴:banghead:

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