This is getting too funny. On my COMPLETELY STOCK 150hp KA24DE, I ran 5w40 year round. Guys, this engine isn't stock.
I would not run the 5w40 if I had a turbo bolted on. And, definitely wouldn't even consider it if I tore the motor down for a complete rebuild prepping it for the turbo.
The KA will run on any weight oil. The question is whether his oil cooler cools enough, and whether he has excessive oil flow/pressure to provide for that cooler, the turbo, and the engine itself. If its a stock oil pump, its pushing its limit.
Older KA engines use 3/4" oil filter mount. Newer engines use the 20mm filter mount.
Not enough info here. So, a 50 weight full synthetic is what I'm going to recommend.