I have been using the Mystik JT4 in my boat for a few months now, and working fine.
I did drain all the oil out except for what was in the injection lines, and the mystik just followed it in and replaced it.
It is not a tcw3 oil. It is for the rave valve applications that usually use the Seadoo XPS synthetic or synthetic blend.
I pay $19 per gallon and can buy it locally. They are made here in houston, and all the diesel maintenance places use their products (not the JT4, of course). I see trucks hauling cases of JT8 all the time, and pallets of the stuff at all the diesel shops.
So apparently, they have a decent reputation.