5w50 and 20W50 have the same viscosity at operating temperature, thats why the numbers are standardized.
The only difference people would see is faster cold start on the 5w50 as its less viscous than the 20W50 when cold.
The cold start part is needed for very cold areas. Engines maintain their oil temperature at 100C or a bit higher, If you are using a 5W40 oil in an engine that is specced to use 0W20 - the operator will suffer from higher fuel costs as the engine has to turn the oil pump through oil that is twice as thick as specced.
If this oil is too thick it may not flow through the bearings as quickly as required and cause hot spots and failure too.
mostly people in Pakistan are stuck with "be awaz engine" syndrome - which is again "ustaad problem" - if you have a lightweight aluminum engine with a DOHC valvetrain it will be loud. Too much noise emanates from the engine.
OTOH,an iron block OHV like an old toyota 3K will be quiet as a mouse, the iron block dampens out the firing noise and also any ringing noise.
btw - A/C tech has come really ahead. You will now see variable displacement compressors like the new corolla uses (which the A/C mechanics immediately throw away because they cannot understand a compressor works without a clutch) - such A/C systems have less than half the load of a normal old A/C system.
Let me examplify my own cars.
toyota camry 2.4 (gadha gari engine) 280,000 kms now - head redone once, block still factory assembled, lubed by cheapo 5W30 bought in buckets and cheapo filters bought in bulk, oil changed every 5000 or 6000 kms. - engine still fit. According to this thread it should have died 150,000 kms ago. How come its still running.
BMW X5 - 3.0 DI turbocharged.
mobil1 0W40 - works sweet for 16000 kms.
camry sounds like crap at high revs compared to an old chevy engine, its just how the engine sounds (it even has foam on it to deaden sound)
BMW too - it makes its own fair share of noise from the engine, but BMW have done other stuff, like add an exhaust diverter for free flowing noisier exhaust, and lots of insulation on the body to prevent drumming noise as most of the car skin is aluminium.
