Sorry to intervene!
Having a lot of research due to Handling GEs IC Nat Gas Engines and attending so many seminars by different lube oil companies operating in the country.....
You both parties are right but i think not getting to make understand each other.
Among various factors there are 3 prime factors which oil manufacturers keep in view while developing lube oils....
- Viscosity index.....i.e how early it reaches all engine parts during cold startup.
2.Evaporation rate and flash point ......i.e. how much its molecular structure bonding remains stable on different ambient conditions with respect to sump oil temperatures.
3.How early its TAN increases and TBN depletes i.e. alklinity decreases and becomes acidic ( The main reason why all manufacturers recommend time frame limit other than kms despite the car is driven 0km).
So specifically talking about the first 2 despite e.g. considering the additive packages same:
For Turbos 0-20W being relatively more thinner is more apropriate as instant Turbo charger lubrication is required along with Main, con rod bearings, crank, cam shafts and other comps.
At the same time it should retain its properties & viscosity at lets say 80C sump temp with high ambient like 50C i.e should be able to withstand evaporation.
To clear the misconception it is equally more benificial in cold and hot weather as compare to any slightly higher grade.
For NAs e.g. R18A /Z, 5-30W has also minus range for cold starting and stable as well during same hot sump and ambient temps. So it is also absolutely fine for NAs....
Thats why both are mentioned in Owner Manual book.
So, for 1.8 it is personal preference to use either of both but for Turbo should stick to 0-20W.
Or i would even prefer 0-40W or 0-50W over 0-20W for Turbo.
5-40W is even more better than 5-30W.....if warranty is not a binding and mandatory.