Any ideas what gear oil might be in my transfercase?

So yesterday after work, I pulled my truck into the shop and did a 14-point checkup on it for winter and my coworker downstairs was checking all the diffs and gearboxes and came up and told me that the oil in my t-case looked like a dark, reddish color and was about 1/2 a quart low so he topped it off with the book spec

$28 Mobil 75w90 Mobil 1 104361 75W-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

Mobil 75w90 synthetic gear oil and also noted to me that it was soaked in oil all over so it's obviously due for a gasket kit in the near future.

But it got me curious as to what the heck may be in there and would it have caused the t-case to leak? I know some truck t-cases do take ATF, but our books say that the ones in my particular 2004 truck take that 75w90 synthetic gear oil and it was an amber brown like any other gear oil you see. I've never heard of a red gear oil personally but is it possible it's the OEM stuff Toyota used? The only red oil I've ever known are ATF and different types of grease. Either way, I am going to be changing it out soon just to be sure but it got me thinking. I hope whatever it is didn't cause any damage to my t-case. It shifts into 4x4 and back to 2x4 just fine.

 

No one?

 

I wouldn't sweat it bt change it as soon as you can just to be sure.