To be completely honest with you neither have I , but the guy who works on my car has. When I had my Gsc S3 cams installed we did springs , retainers, and valve stem seals all at once. It’s all good at least it seems like the op doesn’t have this issue as I also agree that it’s condensation.

 

Valve stems seals are 100% doable with the head on. Pop them off while the spring is off, shove a new one on. They're right there on top of the valve guide, 100% exposed with the valve spring off.

 

Good to know! Aaaand noted.