There is actually a significant difference between the need to replace shock oil vs engine oil. Engine oil is subject to blow by and VERY different forces acting within the engine. They both are subject to condensation which is probably the only real factor degrading shock oil.
Just throwing it out there, I picked up a 2016 Panagale R model back in June. the front end was very harsh, enough so that I was thinking it was over sprung and the previous owner made some changes to them. I just tore them down this weekend and serviced them. The oil looked like used motor oil! with small bits of gunk floating around. The springs were OEM as well so looks like that was the first time they had ever been apart. So I put new seals and changed the oil. Have not ridden it yet though but I'm curious to see if that was the culprit with the front end.
For those of you who will do this service yourself you will want to invest in a cartridge holding tool. The dampening cartridge was installed with lock tight into the bottom of the fork leg and where it is supposed to come apart. It was kind of a bitch to get it apart but with a bit of heat and some muscle it came apart.