Have the exterior of the transmission pretty well cleaned up. Since the internal condition of the trans is unknown, I'm planning to do a fill and drain at least twice before calling it good. Wondering if I should use some type of detergent additive (Seafoam, etc.) with the gear oil for the first fill. I'm most concerned with flushing out any sludge or metal particles. Any advice would be appreciated. 

Filling and draining might be a waste of fluid and money.  If it works well from the beginning then it's probably fine.  I had one that had sat for quite a while, there must have been rust on the cones because it was very grabby when shifting, the synchros weren't working right.  It took a while to wear back in after I finally tried a funky Swepco/ATF blend to use in it.  But it was great once it got a few hundred miles on it.

Maybe start with a cheap throwaway fluid to purge any rust particles then fill it with Redline.  Don't abuse it just run it through the gears until it stabilizes then switch to the spendy fluid.

AC/Delco Friction Modified transmission fluid----

I used Sta-lube gl-4 85w-90 gear oil from Napa 11.49 a qt when I refilled mine


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20 hours ago, ryanotown22 said:

I used Sta-lube gl-4 85w-90 gear oil from Napa 

That's what's in my close ratio right now.

Although many have reported good results with Redline, it didn't work well in mine. IDK why. I'm going to try the Delco Friction Modified gear oil next change.