My JDM 1.5 is good for 0w20 and 5w30 as per JDM oil selectors.
Use this as a reference point for your 4g18 and 4g13 builds https://applications.castrol.com/oilselector/en_gb/c/recommendation?vehicleType=cars&manufacturer=mitsubishi-(eu)&model=lancer-cs0-(2003-2010)&modelType=lancer-1-6-16v-cs0-(2003-2009)
Furthermore do note the 4g13 and 4g18's used in lancer cs were unlike their CK brethren(hell I doubt the ck even came with a 4g18 so you've basically got a post year 2002/2003 engine we're referring here). Aisay tou 4g13 was in our 86 lancer too does not mean the engine remains the same. Hell there are even differences in the 4G15's manufactured and things change from year 2000 to date there have been at least 6 iterations of the 4g15 itself in the same engine same platform and im not referring to 4g15 gdi, 4g15 turbo or etc. woh sab alag hai. I am assuming same minor changes for the 4g13's and 4g18's. The reason I am talking about these is simple, the time frame our cars came in and were being delivered the recommended viscosity was no where close to what you are referring to i.e 15w. Feel free to check the 2.0l and 2.4l powerplants in the lancer us model within the same time frame even they do not have these viscosity suggestions. Please read the following on where Japanese origin engines stood at the period you mentioned - https://noln.net/2017/06/30/skinny-ow-16-oil/
Btw they are already launching 0W8 this year.
If you can please read this it would help clear a lot of your thick oil misconceptions sara na parh sakay tou the excerpt is here for you: https://www.bobistheoilguy.com/motor-oil-102/
"most engine wear occurs at startup, which is also when the oil is too thick. Most of the thick oil at startup actually goes through the bypass valve back to the engine oil sump and not into your engine oil galleys where it is spread to various engine components. This is especially true when you really step on that gas pedal. You need more lubrication, but you actually get less. While there is some residual oil that is lubricating the engine during the start-up period and that oil has additives and can provide some lubrication, but the optimal system would be to have the oil at the correct viscosity at all temperatures. In our example the two oils are the same thickness at operating temperature, but very different at start-up, and even wider apart if the starting temperature falls below freezing."
Lower initial viscosity has nothing to do with api specification, newer API specifications are backwards compatible. Thinner flowing oils at startup are preferred for even our engines whether it be 4g13, 4g15 or 4g18 originating from the lancer CS.