After finishing my last two rides on my second hand bought bike, I've noticed both stanchions appear as if they are leaking oil Could it be just WD residues or excess suspension oil?
Alternatives what this liquid can be include:
Suspension oil (as you found out). Once I had a similar case of a suspension fork leaking oil due to worn-out seals.
Accumulated rainwater / oil mix. Sounds strange, but I had this once with a used MTB I bought in Kathmandu :-) It had a Suntour fork that had a kind of empty upper compartment in the stanchion tubes. Seemingly this had filled up with rainwater over time, which mixed with some rust and oil from cleaning and normal (?) losses. When I turned the bike on its head, it all came out, maybe 150-200 ml per tube. The suspension fork would operate completely normally afterwards.