The most common engine seals that eventually seems to leak are the rear main and valve cover gasket. What other seals would a HM oil help prevent leaks in?
Front/Rear CAM seals
It won't have an impact on valve cover.
Hopefully the valve seals would benefit from HM oils as well and the
front and rear crank seals.
I've used high mileage oils from Pennzoil, Castrol, Mobil 1, and Valvoline for the past 30k miles. In that time, the puff of blue smoke on startup from weak valve seals has gone from significant down to almost non-existent. When I had the intake manifold off this spring I could see a tiny bit of oil seeping down the valve stems, but apparently that's not enough to make any visible smoke when it fires up. I did have a rear cam seal start to weep this winter, but that may have been previously weakened when the pcv system failed and pressurized the crankcase a few years back. I know it puked oil from the top end when that happened.
Don't have good data on overall oil consumption with HM vs non-HM since the car had slow leaks of various nature from hoses and places other than seals up until this year when I fixed the last of the problems. This oci it's using about 1 quart in 5k miles.
When discussing seals such as in engines, transmissions, and other driveline components, the context is that of using flexible, elastomeric materials to keep fluids contained within the mechanical system that have rotating components exiting the system, such as protruding rotating shafts.
For a gasket, the context is that of using a material (such as silicone or cork or composite), or some other material to keep fluids contained in and around stationary objects, such as the valve cover gasket on an engine or the pan gasket on an automatic transmission.
Both seals and gaskets "prevent" loss of fluids...
Conditioning seals refers to the following actions:
Seal swell due to Seal elastomer molecular replacement
I personally don't think they do anything different than non high mileage oils. I personally think HM is just a marketing term. I've never noticed a difference in stopping oil leaks or reducing burn off with HM and standard conventional oil. Never tried HM synthetics but I imagine the results would be similar. Another thing to consider is if HM oils are such a great thing, why are there no diesel HM oils? Diesel engines typically outlast gas engines by almost double the mileage so why don't we see things like Rotella, Delo or Delvac HM brews? Food for thought.
I think HM oils are all on the thicker side of the stated viscosity which may cut down on leaks also. I also believe that conventional oil is always better at slowing leaks than synthetic given the same weights.