AMSOIL Synthetic Marine Gear Lube SAE 75W/80W-90 is what I just put in a rebuilt Bmax HT for break-in. Going with NEO from the Bravo shop after a few hours. A buddy had a 5 gal bucket of the Amsoil and gave me a free gallon of it for the first fill.
Ordered the NEO from John at the Bravo Shop and it was shipped out the same day. Can't beat that for fast service. Drive shifts very smooth with the Amsoil, will water test this coming weekend.

If you have a rebuild drive with new gears any drive, you should not use a full syn oil to break it in. As this is not going to let the gears and bearings burnish together the way they need to, you should use the Merc gear lube for break in for the first 5-8 hrs of the drives life and than switch over.

Waconda,

I have done that with all my other drives but have researched many articles on the subject. All the new car manufactures add the synthetic oil right out of the box but also have a better manufacturing process and surface finishes on the gears then Mercury does (forged XR’s and lower gears). I have also seen a report that compared merc hp oil against some other drive oils including synthetics. The merc oil in many cases did as good if not better than some synthetics.
As you know the most critical part of breaking in new gears is to get a good pattern and yes burnishing them in. In the tests that I have seen and have had done myself, the merc oil has about the same slip as a synthetic but all of the oils are pretty close (independent lab).
One big advantage of a synthetic is the ability to take and dissipate heat without thermal runaway. The biggest problem with the break-in of new gears is the friction of the new gears due to the rougher surface that produces heat and puts it into the surface of the material and could cause scuffing and or so much heat it could affect the case hardness of the gear itself that would greatly reduce the life of the gear.
Typically, good cheap oil for break-in is fine as you are only going to run it for only a few hours and take it easy. But with a single engine 6,000 pound boat just to get on plane puts a lot of stress/friction on the gears causing the heat and scuffing so every precaution to avoid this I think would be in our best interest.
I’m not an expert on anything, just try to investigate all the opportunities to improve the things that we all struggle with and share that information.
I got the Amsoil marine lube for free for helping a buddy on his boat and that is why I put it in rather than spending close to $20 per quart to the merc hp stuff. Next in will be the NEO I purchased from you last week.
I think this could be a good discussion so I would like to hear any other opinions from Mr. Gagets, Artic…

It is interesting to see what results others have had trying to break in gears. I did not see good results going with synthetic oil right out of the gate, it actually made the gears pit out on a couple teeth with in 10 hrs. There are high spots on the gears and they need to wear off before pressure causes damage to the gear and the Bravo Shop oil did not let the gears burnish. We have had the best luck with Merc lube for break in and then switch. I also think it has a lot to do with the type of break in you do also, I'm sure everyone has their method they use.