What transmission fluid do I use?

el40

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I had a nv3500 transmission of a 1998 gmc k1500 5.7 rebuilt and the shop told me to use 10w40 synthetic oil
But in the manual
It says to use synchromesh fluid

what should I go with
 

Nad_Yvalhosert

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If Synchromesh is recomended, Synchromesh is what you should use. It's similar to 5w30 in viscosity, but with different additives. 10w40 doesnt have the additive package so dont use it.
Go with GM's advice, not "some shop".
 

Pinger

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If the oil you guys have access to is GL rated... then GL5 is for diffs etc, GL4 is for gearboxes.
GL5 has anti-wear additives that will attack yellow metal synchromesh parts. It can only ever be used in manual box if the box has steel synchro rings.
 

Trenton

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If you go the Amsoil website, they give recommend a fluid called "Manual Synchromesh Transmission Fluid, 5w-30" for the NV3500. So yeah use Synchromesh fluid like stated above.
 

1989GMCSIERRA

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I would use the Synchromesh but really don't understand the 10w40 in the transmission.

some transmissions use actual gear oil

most newer transmissions use a ATF type fluid. My ZF6 uses Mercon V
 
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