On the Ct 90 forum Shell Rotella 5w-40 is highlly recomended. It is a full synthetic designed for diesel engines. The reason the CT guys like it is it dosen't have the adddatives that can degrade old seals. I used it recently when I changed the oil in both my CT's and it made a huge difference. It is shifting like I just repalced the clutch and adjusted everything and all I did was change the oil.
Robin
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