I just changed my diffs and transfer case oil with Amsoil 75W-90 "Gear Lube". I thought there was a little dirt on the outside of the bottles but looking closely after completing the job, there's a residue inside. I wish I'd noticed this before doing the job. I used a hand pump that draws from the middle of the bottle so likely none of this residue was pumped into the truck. I emailed pictures to Amsoil and they say it's probably additive fallout. The oil had been sitting on my shelf for probably 2-3 years and exposed to some very cold temps. Amsoil says their products have a 5 year shelf life. Should I be planning to drain and refill with fresh fluid or is this really not a big deal? The gear oil I used was in clear bottles which is why I saw the residue. Had I unused their newer, solid grey colour bottles, I wouldn't have seen the fallout. I suppose I could send a sample for analysis before buying another 7 or 8 quarts at $19 each. The moral of the story, don't hoarde oil for too long, especially at cold temps.