: There is a large, cylindrical filtration unit which appears to be quite old and sits between the furnace and the hot water heater. Would this be the source of the problem? If so, what's the solution? I haven't located any of the "filters" on the market or found anyone that knows what they are.

: Dixie Faries
The water that is delivered to the house is perfectly clear drinking water, Right? The piping in this 30 year old house is what? Galvanized? The water heater being turned up high sounds like heat from the burner is rising to the roof surface where is the only place black oily tar material could be encountered. Then what this tarry stuff drifts in the air, down the vent stack to somehow enter the vent tube in the tubs? The filter unit serves the hot water for the sinks and the washer also right? but no oily deposits? The sinks probably use the same vent stack as the tub, but the water stands never so long in the sink as in the tub. The washer drain uses an open vent pipe which won't allow the tar. Assume the tar specks come from the roof agitated by the waterheater heating the roof surface. This is not good, THIS is DANGEROUS! Look at the heat vent from the hot water heater. Can you get to the attic to see it as it passes through the roof? Can you turn the light off in the attic and see outside sunlight shining into the now dark attic? Maybe the answer is a little Henry's tar and a metal roof jack to go over the vent pipe. It sounds like the roofing cement is hard and brittle and cracking, and when the hot water heater is very hot the roof jack expands, letting little bits into the vent. Sounds very unlikely that it could traverse to the tub vents, though, because they are generally protected by a trap. Could have built up with time though. Have you had a roof job done recently?

The fresh water pipes are a very unlikely manner to get these oily deposits, but what the heck, open the filter cylinder thingie, pull out the filters, and look inside with a flashlight. Any black oily stuff? No? take the filters to the plumbing supply house (Not Home Depot) and ask for replacement filters. Take the name and model number of the filter housing with you.

How about the caulking and walls around the tub. Is there any chance that there's tarpaper behind the wallboard? old tarpaper could be leaving your oily spots . Do the tubs share a same wall? Maybe thats where you need to look, is behing the wallpaper and drywall to see if the tar substance comes from there