If your loosing oil pressure it's not normal and 10w40 should be plenty to keep it up. Put a mechanical gauge on the motor to verify pressure is dropping and if it is then there is a problem.
Similar happened to me on my 4 stroke seadoo. Startyed running rough and missing and thought it's a 4 stroke, should not need plugs. After other things that's exactly what it needed
I've done the replacement and know of no way to do it without removing the drive and bell housing. The motor does not need to be removed but then again I was able get to the back of the connection even though very tight
If you can remove the fitting that connects to the raw water pump hose and...
Unable to say why it's so close, but if it's Sierra it could be product issue. So long as its not touching it should be fine. If it does barely touch it should be no issue with it being plastic
I go to BAM and enter serial number on top right. If valid number it shows up and just select it and it takes you to parts list. Can also look thru by finding motor/drive and by size and year
https://www.mercruiserparts.com/mercury-marine-parts-lookup-drawings
The link shows
Alpha 18643A5...
Get a short piece (4 to 8 inch) of clear hose from the hardware store and a double barbed fitting and hose clamp. Disconnect the hose at the thermostat coming from the drive and put hose in between. See if you see bubbles with motor running
Use the dipstick
Many motors do not have a dipstick tube which goes down far enough to get most the oil out. Would have to use smaller plastic tube down dipstick tube so it reaches further. Either that or use drain pan plug and remove it
180 to 200 is way to high. To be that high the gauge is not accurate
If the test was done cold and not warmed up, this will cause issues also.
Unless the "Mechanic" (so he's called) also did a leak down test I would ask just how does he know the possible issue is the head, the rings or just a...
