How many hours will a #6 drive last before it need to be refreshed behind 700's . Does 370 hours need refreshed or will it last for another 150 hours.

I'd say it all depends on how they've been maintained and how old they are. My advice is to have them checked out by a reputable drive specialist. The M6s on my 2009 36 w/ 700s w/ 200 hours on them needed full rebuilds when I purchased the boat in 2020. However, my boat previously had 1075s in it for the first 2 years when it was first built, so that definitely added unknown hours and high HP abuse that could've added to the severe wear we saw. I recommend Mike Holford at CK Performance in Michigan if you can possibly get your boat to him - he's one of the best in the industry with the big Merc drives and he treated me great with my rebuilds.

I took apart my drives after 450 hours to do preventative maintenance. Could seem some wear, but had a few more seasons left to go. Stage II 850 hp.

There are guys with 600 hours still going strong. Obviously predicated upon use, but my personal experience was 200 out of the 450 were hard hours that I put on them. YMMV.

So it seems like it can be hit or miss on the life of these drives.

yep just like a bravo

I'd say mild horsepower and regular maintenance and they'll live a long time. I changed my gear lube twice a season and it would look the same going in as it did coming out.

Big power and its significantly shorter.

I don't think you'll have any issues getting to 500 hours

Multi-grade gear lubes can get sheared up and damaged, reducing their capacity to maintain a film strength. Good call to change it out often. Have you considered getting a fluid sample tested?