It’s OK For Struts To Weep A Little. It’s Not OK For Them To Leak.
The dust and dirt near the top of the strut (where the seal is and the rod comes out) can be damp with oil and yet the strut does not require replacement. What you don’t want is for the oil to be visibly leaking or actually dripping from the strut. That is a strut that should probably be replaced soon.
Did you have a look ? Did you compare it with the other strut(s) ?
CSA