Marine: Boating, Sailing, PWC, GPS and Navigation - Noise lower unit

I have a 2001 Mercury 2 stoke, 90hp that is noise like a thrash machine when dropped in gear at ideal. Two different dealerships tell me this is normal with a stainless prop on this motor. Another said I could try synthetic lube to quiet this problem but they did not reccomend a brand. Has anybody experienced this problem? Has anybody run a sythetic lube in thier lower unit and if so what brand?

Does it grind just going into gear, or all the time?

If it's just when it's going into gear, that IS pretty normal, although slop in the shift mechanism may enhance it. A crisp "slam" into gear (at idle), rather than a slow slide will reduce the gear grind.

If it's grinding all the time, then you've got problems with the bearings, and/or gears. Bearings will give off a "rumbling" sound when in gear.

Mercury makes an excellent synthetic gear lube. It's costly, but after more than 5,000 combined hours on our 2 patrol boats, we've never had a lower unit failure.

We change the gear lube every 100 hours, which is just about monthly for us in the summer. I'd recommend the average recreational boater do it in the pre-season spring maintenance, and once again mid-summer. More or less, depending on how many hours you run it.

Seeing that the noise sounds worse with a stainless steel prop it is probable bearing noise. Hopefully the synthetic lube will quiet it.Thanks