I did engine and drive every season and impellers every other.

impeller every year? every 2 or 4 years? you have no idea how many years on that impeller. replace it. pull hose on inside of transom. backflush coolers from pump back to transom. put another hose on transom. backflush from there out to the water pickup(s). NOW you can worry about oil. and belts too...

Thanks for the feedback gents.

The previous owner had a local shop do all the winterizing and de-winterizing every year, and they put in a new pump and impeller last spring. I'm not opposed to dropping a new one in.... Is this something I can order from amazon?

This kit here: https://www.amazon.com/Impeller-Serv.../dp/B0077RKSJM
??

I'm considering Mobil1 15w40, vs 15w50 as this is mostly in cool weather climate here in Montana. We do hit the mid 90s here in July/Aug, but the lake water is rarely much above 60. 50 weight oil just seems kinda heavy??? ...my mind isn't made up yet, just asking the question.

It looks like I have this:
https://www.walmart.com/ip/Mobil-1-1...354#about-item

Or this as an option:
https://www.walmart.com/ip/Mobil-1-1...5-qt./20713647

I believe this has the high performance quick silver or mercury equivalent gear oil in the LU, I think I'll go with that. I'll get ready to cut an arm or a leg off for that one.

Now I just need to figure out what to run as a filter. I'm hoping to pick up everything today so I can get it done tomorrow.

Is purpose here is to clear out an gunk that may have made it's way into the coolers, or the stern drive pickup?

Yep. If i winterize my boat I backflush everything just for kicks, and it's especially a good idea on an known boat so if a problem develops you've eleminated some possibilities.

I also rarely get to go full tilt and stay with the 40 weights. No problems so far. If I do get to run wide open for a while it'll drop to 25 or so at idle then quickly regroup back up. If you don't see it regroup after some running hard then you can try the thicker stuff.

There is also nothing wrong with 25 at idle. And I hear full synthetic has a film strength much higher than, well, oil-based oils.

So you're saying if you do a lot of WOT operating, you'd want thicker oil??

Sorry, I didn't want to turn this into an oil thread :). I live on a pretty big lake, one of the spots we go to often is about 20 miles of boating, so if the water is decent we just drop the hammer and get there, running 30 min straight sometimes, so, perhaps I'll go with the15w50.

Get the Mobil 1 15w-50. It is the older and better formula.

So I got all the plugs back in, tightened it all up and gave it a test start to make sure everything was okay before i started changing fluids. My partner was supposed to be watching water flow. Well he didn't take notice that the ears slipped off of the pickup.... i noticed engine temp start to climb and shut it down. Didn't take long, the impeller WAS working, it isn't now.

So, I guess I'm replacing it :). The impeller with some other parts (some sort of back plate) is about $70. The shop wanted to 2 hours labor to do it themselves, and they're charging $130 an hour, so I'll be doing this myself. This cant take 2 hours to replace!!??