I run the HP Merc/Quicksilver oil in all my cases.

2 main reasons why:
It’s $9.95 at Walmart
It mixes well if you have minor water intrusion

That is correct for Merc gear oils vs Sierra gear oils are made by 2 different companies and with different additives. None are the same

To add some other info. Merc has 3 different types of gear oil and even those 3 merc gear oils are made by 3 different companies.

Citgo makes the Premium Blend merc gear oil --- Bel ray use to make the High Performance Merc gear oil but currently that has changed is what I am told. And the Merc Racing gear oil - I do know who makes it, but Have to leave it at that. Its made here in the USA .

There are dozens of threads on here but most of them come up with this as a good, reasonably priced option. Mystic JT7 85W-140 One of my favorites.

Amsoil Severe Gear was once an option but sucks on the cone clutches.

We use the AADS oil...neo? Whatever it’s called.

Used Amsoil Synthetic Marine Gear Lube in B1's and they were noticeably notchy when switching from neutral to either forward or reverse. Next season Merc. HP put back in an silky smooth again.

Apparently triple engines boats are proned to break center drive... been using the gm oe synthetic 75-90 gear oil for 5 years and no issue! Its a sport cruiser more than a offshore boat beaten to death, i agree, but works fine for me!

What's an alternative to use in a #5 SSM ? I typically buy the Merc Racing Gear oil in 5 gal containers, but HOLY $$$ ! It's a few hundred a pale.

Any quality alternative there? I'm not worried about clutches, but most concerned gears are bathed in high quality lubricant.

with oil dropping to all time lows..they should be paying you to use it!!!

I was told by Teague while purchasing one of there Platinum XR drives that they switched from AMS oil back to Merc and saw better wear characteristics. That being said I've also heard really good things about Rev-X Drive-H oil and will be trying it this season.

I've switched from Merc HP to Merc Racing 90w and buy it 5gal pail for $276. Works out to $13.80 per qt. It does not leave sludge in the drive reservoir and does not need topped off like the Merc HP needed.

I've ran Mobil 1 Synthetic 75w90 before in a drive that had leaking seals. When the oil in resevoir bottle changed color, drive was torn down to repair and clean. Lower gear faces were allready pitted and needed replaced. Merc oil is labeled as being able to run with water intrusion to a degree.