Consider the use of 5o weight transmission oil for use in heavy truck transmissions. early 13 speed trannys used mineral oil but later specifications use 5o weight motor oil. delvac makes an excelent transmission specific 50 weight that i have used for 950000 miles in my eaton super ten. I belive that 50 weight motor oil and 90 weight gear oil Viscosities are the same. Good luck.