I use regular Vaseline, plumbers o-ring grease or a silicone grease for potable water use. The Eheim o-rings are not pure rubber so Vaseline doesn't degrade them, at least it is recommended by Eheim to use without issues.
Wipe the grime off the o-rings with water and a damp rag. Check for nicks or tears in the o-rings. You only need to apply a light film of lube. I apply a dab to my finger and then slide the o-ring through my finger and thumb.