Picked up a w115 300d car that had been sitting for awhile and the rear doors were very stiff, hard to open without great effort. Take a heat gun and move it up along the hinge barrel. After some time, the dried out congealed grease inside will start to liquefy and bubble out of the hinge joints. Wipe away old grease and take your needle tip attachment and insert in hinge grease port and pump fresh grease into door hinge and then work door back and forth, and voila, like new again!