A twin lubrication oil tank filling system with a single oil fill port that prevents overfilling of both a primary oil tank and a secondary oil with a gravity fill system by directing oil primarily into the secondary oil tank by means of a special oil diverter, a float valve for the secondary oil tank that closes when the oil level in the secondary tank reaches a full level to cause additional oil flow to pour into the primary oil tank and a float valve for the primary oil tank that closes when the oil level in the primary tank reaches a full level.