If HM oil cost the same as regular dino, would there a reason to use regular dino?

Basically, are there any drawbacks to HM oil besides price?

The answer probably involves some semi-esoteric chemistry...

I think the use of HM oil is more for cars that were never fully taken care of since the first mile on the engine. If you've always taken good care of the oil, done proper, regular oil changes. The need for HM oils is slim to none.

I've always thought most of the HM oils are on the thicker side of their grade and most don't have the API Energy Conserving designation.

Better MPG is my guess.

From what I've read, there is no downside other than the lack of the energy conserving label in running a HM oil on a new car.

I believe MaxLife is recommended for new cars as well.

My understanding though is that HM oil contains a seal swelling additive that can probably cause a seal to weaken over time.

This was my thought as well so comparing mobil 1 to mobil 1 high mileage it may be better to use regular mobil 1 to keep the seals in good shape longer. Though I think most synthetics also have a seal swelling additive just maybe not as much as high mileage oils.

Most modern HM oils use a seal conditioner. Doubtful if any use the same level and harshness of what you call "seal swellers" from the dark ages. Remember Valvoline and a few other brands are issueing warranties for 2 to 300k on engines, that at the time of registry may have close to high mileage on them already. I'd bet some pencil pusher in accounting thought of that issue coming up.