Tar often mixes or softens with oils because many tar components are oily themselves. Oil can make tar more fluid, more penetrating, and slower to dry.
Traditional tar finishes are sometimes associated with oils because both materials can protect wood and both resist water. Linseed oil, for example, has often been used in wood finishing, while tar adds dark colour, smell, and water-shedding behaviour.
Oil can change tar by reducing viscosity. A tar-oil mixture may spread more easily and penetrate rough timber better than thick tar alone. The trade-off is that it can also remain softer longer and may darken or stain more deeply.
Compatibility depends on the oil and the tar. Drying oils behave differently from mineral oils. A product already blended for wood treatment is not the same as an improvised mixture. Too much oil can weaken the surface film or create a finish that stays tacky.
The main behaviour is softening and dilution. Oil can make tar easier to work into a surface, but it can also lengthen settling time and reduce the firmness of the final coat.