Once Bajrangi were involved in some work for lord Ram and Lord Shani dev appeared and as his nature tried to created some obstacles on it . Hanumanji warned him and told him that it was urgent but shani dev didn’t care of his voice. Hanumanji caught Shani-devta in his tail and at the same time He continued doing his this Ram karya through jumping here and there. Lord shani was not able to protect against him and got wounded badly.
When lord hanumanji finished his work then he released him . lord shani apologized to Hanumanji and promised that he never ever create obstacles in Shri ram and hanuman work .
And lord hanuman ji devotee would get special blessings from shani dev. Listing this Lord hanuman was very happy.
Lord shani asked some oil so that he could put on scars for recover and hanuman ji provided him. After that lord shani was very happy and promised to universe that if any devotee would present oil on shani dev, wishes comes true.
Thus devotee of shani dev offers mustard oil to lord shani dev and get blessings of him .
According to an ancient story overambitious Ravana invaded over Lord Shani and imprisoned him in lanka.Lord shani remained till Lord Hanuman reached there.
Once lord hanuman entered in lanka in serach of goddess sita . he got message from her and caught by army of ravana. But after some time he released himself and set fire on lanka. He also found lord shani dev in the basement of palace . he reached near him and released him. Lord shani dev thanked him heartily. He also promised that he may not become trouble those who are the devotees of Lord Bajrangi.