First, thanks for the replies- I'm trying to get a handle on what happened and appreciate the input, but here's my response: First, I've done a lot of research on oil and temps. Bottom line- what I've found is that tests show synthetic oil can withstand occasional temperatures in excess of 300 deg, but the change interval should be shortened. The oil in these engines were exposed to those temperatures AT MOST 3 or 4 times for a period of no more than 1 minute each time. I changed the oil when I swapped out the oil coolers in 2013 and the oil temps never exceeded 260 after. I'm also not convinced that bearings will be damaged by exposure to 300 deg once or twice for a short interval. As to the oil lines: yes, I meant 3/8 but I'm talking ID, which is what Mercuriser stock engs come with. No one recommended these lines, I just reused the Merc parts. For the record, there was no engine temp/ cooling issues at any time, Timing and A/F mixture wouldn't cause bearing failure and they were never out of spec. The oil lines were absolutely with 100% certainty plumbed correctly.

I'm just a shade tree knuckle buster with mostly just googling skills and can't even haul gravel, but I've read plenty of problems here using only -10 lines with anything above 500hp. Remember that when those bypass valves are in use it dumps hot unfiltered oil back into the pan. More people have ruined a new motor with the stock bypass valve than hot oil here.

A high volume pump against stock lines may open that bypass valve and circulate unfiltered oil through the engine.

a lot of people THINK the 10 lb bypass ruined their engine,i know of several gen 6 marine engines with a 10 lb bypass that have run for years.just saying!

Who's oil filter you useing

So I get that sending some oil through the bypass un-cooled will raise the overall temp of the oil as measured by the thermostats located at then end of the first remote oil line, but the highest temp was never in excess of 260 deg, usually closer to 220 (except for 3 or 4 times for short periods before I changed the coolers). That means oil temp was not an issue and I still say that recycling unfiltered oil through a new engine for 90 hrs will not destroy a set of main bearings. I'm well aware that marine engines are different animals than car engines, but as a reality check: 90 hrs X 50mph = 4500 miles.

Post som pictures of your bearings. My thoughts on the bypass mode One something starts to fail it snowballs.

I started out using Mobil 1 filters and then switched to K&N. Used Amsoil 20w-50. With all due respect, I think the problem is not inferior filters or liquids, it takes something much more fundamental to cause a mechanical failure like this at 90 hrs.

My point is that cheap and small filters collapse inside shutting off oil flow and go into bypass mode and then you die.

well you don,t die,the engine dies and you live on to fix it,lol.

I only pulled the caps off and saw the #1 (forward most) main bearing on both engines. Both bearings had very visible wear. On one the surface was mostly worn down to the "copper" under-layer, on the other the outer silver colored layer was still intact but had may ruts running around the circumference of the surface.