I serviced my diff after the first tank and filled half way and worked the oil in.
After several gallons of fuel I took it apart last night .
Although there were no visible leaks (And my diff case was fully intact) the oil inside was minimal and very black.

Many suspect that HPI doesn't put enough oil in their diffs from the factory.
From the factory I think they fill them half way bit after turning the gears and all the little crevices getting oil it looks like a skim.

The gears were near new looking.. I cleaned them all up, reinstalled, added 5,000wt (had no 1,000) and she is all ready to go.

I did notice the tranny case had some sand in it, this apparently is getting in at the top end seal so although the upper shaft is a floating shaft I reinstalled the shaft seal, added a bearing and reassembled.

More to come in 6 months when I take my diff apart again (unless I break something)