I recently switched my '09 Forester to M1 and it started consuming oil, but I'm sticking with it (just checking the oil level when I fill up gas), to see if the consumption goes down. If after a few oil changes it continues to consume M1, I'll switch to something else.
Have you previously been using synthetic oil? B/c synthetics tend to clean better, changing at high mileage can sometimes result in the oil cleaning deposits that were previously preventing leaks/consumption.
M1 HM is a thick 30wt though, it's not thick-thick, but it's thicker than the norm (most HM oils are thicker than the norm for that weight).
If you're thinking of going to conventional, I'd suggest first trying Valvoline MaxLife for 2-3 oil changes to see if it reduces consumption. MaxLife is a semi-synthetic, it's a HM oil so it's thicker than normal 30wts, it's got seal conditioners to help restore seals that might be causing leaks/consumption, and oil analysis done on it show it's a very solid oil that protects engines well.
If after 2-3 oil changes the consumption is still high, than you can either try to fix the problem, or see about moving up to a thicker oil (Rotella T6 5w-40 is a pretty sweet 40w, though that's going back to synthetics).
Also, do you know if the PCV valve has been changed, might want to check it to make sure it's clean and working. If it's busted this could cause some oil consumption issues.
Lastly, check out BobIsTheOilGuy.com it's a great oil website, and you can find a bunch of info on MaxLife and other HM oils there.
(and just to make sure it's stated, I'm not affiliated wit Valvoline, Mobil, or any oil company, and my only relationship to BITOG is I read and post on their forum)