Midrash su II Samuele 22:29

(Exod. 27:20): CLEAR OIL OF [BEATEN] OLIVES. By universal custom, if one has bad oil, he burns it in a lamp; but the good <oil> he puts in cooked food. In the Temple, however, they do not do so. Rather they put clear olive oil into the lamp, and that which is second (in quality)¹³On the three grades of olive oil, see Men. 8:4; TMen. 9:6. they put into the meal offering. David said (in II Sam. 22:29): FOR YOU ARE MY LAMP, O LORD….