wd40 is mostly fish oil and a few cleaning solvents.
It was originaly designed for water displacement(WD) for military electronics.
The fish oil is a short term lubricant, but dries out over time. eventually becoming tacky/sticky if enough is used.
your case will look nice and polished at first but will eventually dull and attract dust/dirt
I've always cleaned my PC's and laptops with IMS (industrial methylated spirits), that's the clear sort and not the sort with purple dye in to avoid smears, it's probably the wrong thing to do but it's never done my stuff any harm, I just test it in a non visible place first to check it doesn't react with the plastic/paint