I've got a case of 20w50 Golden specto, and a case of Castrol GTX 20w50 both from around 1990 with SG rating. I'd like to get rid of this oil. My motorcycles are 1978 era so SG should be ample? Also 20w50 is a little thick, would mixing a quart of 10w30 to 2 quarts of this old stuff be advisable? Or should I just throw it away?