I have a customer race car that needed to be resealed at the intermediate plate.
Now that it's back together I'd like to top off the fluid level.
The gear oil in the box is red and doesn't have a strong gear oil smell.

I asked the customer if it's from Redline or Lubrication Engineers.
The response he got from from his previous shop is - Swepco with a little ATF.

I think I've heard of that mixture before but not sure of the application.
The gearbox has a Guard TB diff. Is that a recommended additive for the Guard diff?
Is anyone else adding ATF to their 914 gearbox?

I haven't heard of it

we have used an additive for 15 years
but its BG - MGC

I haven't heard of AFT

I have a friend that uses STP in his 915. He swears by it. He probably does close to 20 DE/TT a year. Engine is a stout 3.2 making 270hp. Trans has no limited slip. I can't remember what gear oil he is using.

ATF - Automatic Transmission Fluid

Yes
Fat fingers on a phone keyboard
I was trying to say I haven't heard of anyone using atf

Hadn't heard of using ATF as an additive before.

I did a bit of research recently for a Hewland transaxle that has an LSD, and was told that at least for racing applications that friction modifiers are not necessarily needed if you are using synthetic lube.

I think I've heard that the friction modifiers increase the slipperiness to optimize the clutch action in the LSD.

Maybe ask the LSD manufacturer what's recommended?