I have been using Amsoil Marine with Lucus additive in my XR Drives for three years, changing drive oil every 25-30 hours with great results. I was recently given 5 gallons of Amsoil but did not notice until I went to change my oil that it was Severe Gear and not Marine. I asked several people if this was ok and all said it was fine. Second time out with the boat and noticed my starboard drive had a hesitation of about 10 seconds going in both forward and reverse (makes parking kind of narly) Anyhow my question is... Is it the oil or just a coincidence? Obviously the cone clutch is not engaging so is this a repair bill or change back to the Amsoil Marine?

how many hours on drives...?

cheapest option would be to switch the oil out, cable adjustment, and cross your fingers...

otherwise sounds like cone clutch...

99% sure its the lube. For some reason, some drives don't like some aftermarket gear oils. The marine prob has or doesn't have one of the additives in the severe.
Geno's drives on his AT had one of the Amsoil gear lubes in them when he bought it and he had the delayed shift engagement. He switched to Merc and the delay went away.

+1 on what Griff advised... seen it, experienced it and as a life long Amsoil consumer, I never would have thought that the advanced lubrication properties of the oil would be a detriment to how a B-drive shifts but it does.

For the cost of the merc gear lube, change it out and test the results - still cheaper than diagnosing by a Merc tech.

For a definite answer on the difference between the severe and the Marine Amsoil, you would have to follow up with Amsoil Tech, but, as I try to remember my conversation a few years back with them, it had to do with an additive in the Marine that help keep water in suspension at a higher % of volume before emulsion (milkshake), higher anti-foaming and anti corrosion detergents - lubrication properties were nearly identical based on oil weight ( 75/90 )

I experienced the same 8 second delay years ago when I switched to Amsoil Severe Gear oil,, switched back to Merc. oil and it shifted as normal,, so I removed the cone clutch and it was extremely worn down,, it proved to me Amsoil is so much slicker than Merc. oil .,,great protection for all the internal parts.