A good 5W-30 is no thinner than a a typical 10W-30 at operating temperature. That can be seen in the second viscosity graph I posted above.
At cold start up - All oil is many times thicker than at operating temperature. That can be seen in the first graph I posted.
Bearings - can't help help you on the deep scratches. I'd be questioning that too. If you check again, get some plastigage. That will tell you the bearing clearances - and that's what will determine the need for higher viscosity.
Personally, I wouldn't keep dumping cleaners and thickners in. Let the detergents in your oil clean it up a little bit each time you change the oil.
Calcium is usual an indicator of detergent
https://www.blackstone-labs.com/underst ... diesel.php
15 psi at idle isn't bad. 10 psi woul worry me too. I think 30 is high. BUT. The FSJ sending unit is right at the oil pump.
In any event the damage is going to be under load. So to really answer the question you need to know temperature, rpm as well as pressure.
Barring that, and if your set against 30 weights then for easier starting look at the synthetic 5W-40 and 10W-40 with low pour points.
Here's one of kendall's thats CK, Ck, CI and SN
https://s3.amazonaws.com/phillips-prod/ ... 1508336303
Rather different than Castrols GT1 TDT
https://msdspds.castrol.com/bpglis/Fusi ... 9VX5VU.pdf
here's Shaeffer's
http://www.schaefferoil.com/documents/244-9000-td.pdf
If you want to put those viscosities into a graph like I did in my earlier post:
http://www.widman.biz/English/Calculators/Graph.html
A real good overview of oils is the pdf on his corvair site
http://www.widman.biz/Corvair/English/Links/Oil.html