Who You Want To Follow, Proton "technicians" Or World Class Lubricant Manufacturers?

I read in the Proton Persona manual book where recommended Engine oil is 20W-50. When I gave the oil to my mechanic which has 34 years experience he raised his eyebrows & told me that lighter/thinner oil should be used for my car which does not have any engine problem. I told him that it was stated in my manual book but I myself is skeptic about the correct viscosity recommended by Proton.

Then recently, I found that LIQUI MOLY (of Germany) & BARDAHL (of United States) both recommended 5W-30 Fully Synthetic oil for Proton Persona 1.6 engines. I have bought BARDAHL 5W-30 Fully Synthetic oil and will get it changed shortly.

I dont recommend 5-30 for campro powerplant as the engine will be quite noisy as its too thin.
10-40w semi syn is good enough.

for me its easy, 30 is smaller engine kei-car
40 bigger new engine.
50 bigger older engine.

Well, I have just changed to BARDAHL Fully Synthetic 5W-30 oil and added TUFOIL Engine Treatment oil after flushing the engine with LIQUI MOLY Engine Flush. I also changed to NGK IRIDIUM IX Spark Plugs TR6IX 3689 and K&N Pro Series Oil Filter. Drove the car at 140km/hr.

Found:
1) Engine runs very smoothly & quieter than before oil change.
2) Acceleration is superb.
3) Engine temperature is slightly lower than before.
4) Pick-up is excellent.
5) Gear changes are great and has no lag (same as before).
6) Engine is very responsive.
7) Lastly, I cannot find anything to complain.

Thanks.