I have not used break free as a patch lube. Try it and see. I doubt it will turn into "TAR".
The notion that petroleum and ML's do not mix is myth. I am not sure where the tar idea came from. I have never seen it. I normally use petroleum products to protect the the barrel. Maybe someone was using automotive grease as a patch lube????
Petroleum is not taboo for ML firearms. Ballistol is a favorite and it is mostly mineral oil, (petroleum). I have used drug store mineral oil on patches with no ill effects.
For range shooting the consensus is for a wet patch with something that contains mostly water. For hunting Mink oil is popular. I have been experimenting with tallow, lanolin and joboba mixes with no particularly notable results, only that ramming tight loads is much easier. I have used petroleum oils, like ATF, with nothing remarkable happening. I doubt you can seriously mess up patch lube no matter your choice.
As far as preservative oil or grease, petroleum is an excellent choice. It will not poison your barrel. Leaving a bunch of oil in the breech will make your first shot of the day a problem. If you do that it may mix with fouling to make gunk. Maybe gunk is the same as tar? I bet sperm oil or olive oil would do exactly the same thing.