Oil bath brake additive for gear lube?

Gulf of Alaska
I am working on Geenie GTH1056 telehandler forklift with Spicer 213 axles. I just overhauled one of the axles. It has wet brakes that are lubed by the diff oil. The Spicer/Dana axle manual calls for SAE 80W90, GL4, MIL 1205, with "additives for oil-bath brakes".

My oil distributor (Chevron) is telling me to use limited-slip gear oil, & the Geenie dealer is telling me to just use posi additive from the auto parts store. The axle holds six gallons of oil, so like six tubes of posi additive?? I have never seen a rear diff in a truck that holds more than 5 quarts. Neither of these sounds correct to me & the last thing I want is for the brakes to squeal or be grabby.

What say you?