It would be huge news if M-1 was truly a "bad" synthetic. In my mind, and in my experience it's an excellent product. As is Pennzoil Platinum, Castrol Edge, Valvoline SynPower, etc, etc. In my Semi Truck, I switched from Delo 400 SDE to Phillips 66 Guardol ECT, Because my local Phillips Distributor sells it to me at a cheaper price per gallon than Walmart charges per gallon for Delvac Super 1300, Delo 400, or Rotella T4. All of these oils, including Guardol ECT, meet Detroit Diesel's 93K222 specs, so I'm going with the lowest priced HDEO made by a major company.
I use Mobil 1 HM in my '97 Ford F-250HD 4X4 Pickup with a 460, but I will be switching to Phillips 66 Guardol ECT 10W30, once my M-1 HM stash is used up. Going from a full synthetic to a semi-synthetic doesn't worry me in the least in my 460. I've used practically every name brand you could think of in it, conventional, semi-synthetic, and full synthetic, and other than a tiny leak in my rear main seal, it uses zero oil between changes.
I use Pennzoil Platinum 5W20 in my wife's Dodge Journey, but my Phillips Distributor offers Kendall GT-1 full synthetic 5W20 at a cheaper price. Both oils meet Chrysler's MS-6395 specs.
And I believe the same is true with conventional motor oils; I look first at the specs/approvals. If it meets the engine manufacturer's specs, I'll buy the lowest priced one.
The reason I feel good about doing this is I don't extend the drains in my cars or my pickup, and I typically don't extend the drains in my Semi Truck by a huge amount.
I do choose to use synthetics in my wife's Journey, and my daughter's Sentra,-which gets M-1, because both of these ladies have lead foots, and that concerns me during our cold winters. I figure synthetic will circulate a little faster on cold mornings.
Just my two cents.