D-limonene is "citrus solvent" in its pure form for oils it is stronger than odorless mineral spirits, about equal to turpentine, and weaker than tolulene. The strongest around is an equal mix of toluene and MEK, if you have some old dry grease cake that the others won't soften a day soak in this will. Mild heat helps.
Above that you go to nitroamines for varnish like deposits.
Some cruds respond better to semi-polar solvents like acetone alcohol and MEK, like dumond tech.