The thing about real leather is they LAST. pleather on the other hand doesn't. The texture is about the same for real & fake, however fake leather rely on a ultra think coating to make it feel leather like, this coating will lose it's adhesiveness which result in what we call cracking/flaking, especially at points where they are folded or points where stretch is applied. I think oxidation & humid & heat will cause the layer to lose the adhesive. By applying a leather protector it forms a coating on top, thus reducing the rate of degrading, it also gives a shinny look whether you like it or not. I do it on every Medicom figures that uses somesort of coating for their clothes, whether it's pleather, rubber like tight suits, or plastic like suits that i think will have problems in future.
Those older figures that i didn't, or forgot to end up flaking 1st. Even Jacket on the Rocketeer 2.5 flakes and it feels like high quality pleather.
Just try to avoid leather protectors with alcohol/ethanol contents.
Regarding the die cast parts, always apply car wax on them, even glossy plastic/rubber parts can be used but test 1st just to make sure the wax doesn't react with the paint and wipe them off (some paints come off with alcohol rubs esp panel lines). This will give a real shinny look & also protects the paint from turning dull (esp chrome parts), if you want to remove mild scratches car polisher works, some wax even has UV protection, metal will contract & expand as temperature changes, this will cause the paint to come off. Also best is keep them in dust free environment like a weather sealed detolf and turn on the aircon during summer, buy a dehumidifier if it's too humid (>59%). Also watch out for molds in humid environment, an easy way to tell if you have them is to look at the transparent walls of the boxes, you'll see brown spots on them if you have, all the best getting rid of them...
The rubber tyres will tend to become sticky after few years. Make sure you rotate the tyres or else it will deform due to prolonged pressure on it, putting it on other plastic/rubber surface will cause reaction among the tyres and might "stick" on them.
But if you have kids playing with them, applying any protector or wax is not a good idea unless they wash their hands everytime they are done playing.
As for Kaneda's jacket, it's not REAL leather. I'm not sure putting any oils or anything on it is a good idea...it could melt it or something! So far, I see no cracks in his outfit, or the bike's seats or tires. I get a cloudy oxidation on some of the die cast metal parts and that sucks, but that happens on some of my Transformers too and it SUCKS. I think it's cause of Japan's humidity.