The problem with WD-40 is that it is a very light oil, with a very low viscosity. It is great for short time applications and for rubber to metal applications, but not so good when you need the lubricant also to act as a sealant, preventing dust and dirt to join the internal parts of your marker, to keep your O-Rings properly sealing what they should be, and keeping metal with metal friction. For this applications you need grease, not very liquid oil that will evaporate in some hours.
Also, I know a bit about the effects of different compounds on O-Rings. Motorcycles got a lot of O-Rings on the chain, and there are very useful tests to check the effect of some of the most common oils on them (I can post it if you want) and WD-40 was far from optimal to rubber O-rings.
But at the end of the day, it is your marker and you decision.