What is this yellow fluid in my sump pump?
Well, if it's from the late 80s, then it shouldn't contain any PCBs.

That liquid is probably a dielectric oil with a high viscosity. That would explain why you were thinking it's a solid. They're non-conductive mineral oils and as oil does, it displaces water.

Treat it like a motor oil spill. Here's an MSDS sheet for Shell Morlina oil which is probably similar.