Several of the shock manufacturers use hydraulic oil that is tinted red with the same dye.
If it is diesel, then they could have been trying to get a smoother ride without actually changing the valving.
You can use dexron3mercon or aw32 hydraulic oil as they are the correct viscosity.
If you notice the ride being stiffer after getting actual oil in them, it would confirm my guess.
I can get 5 gallons of aw32 hydraulic oil cheaper than dex3mercon and way cheaper than commercial shock or fork oil. The dexron 3 mercon just has more detergents and will foam faster.