I have been changing the red Allison Filter at about every 12,000 to 15000 miles or so, maybe its over kill. I guess its only money, at one time I was getting filters for 10 bucks or less from an Allison Dealer. I have always only seen the gray looking residue when I wiped the magnet off like you guys mentioned. I am approaching 80,000 miles on mileage and I had switched to Transynd at 48,000 miles so I should be good there for a while, which would be years down the road at the rate I drive, mileage wise. I bought an extra gallon of Transynd to have when I change the filter or need to add any fluid.
Oil changes are once a year due to the low amount of miles that I drive. Actually since I retired I am driving a few more miles since I get out and loafer a little more.