My 2009 LMM has 160K miles and will do 18-20 empty, which is a rare occasion.

Usually its pulling a 3K# empty trailer, or 14-16K# with the boat on. For the last 120K, my overall average mileage is 11.2 all combined usage, including winter snow plowing which is a real MPG average killer.

I treat with Howes, tend to dose it a little heavier than it calls for, but I don't use it in every tank. I'd estimate 25% treated - I tend to buy a jug and then treat 3-5 in a row, then go a few weeks without. Makes sense to me, as a cleanup routine.