Hopefully you are not talking about straight 40 wt oils, but a xW-40. My 93 GCL 4.0 never saw anything less than a 15W-40 from the time I bought it (1999 with 55,000 miles) til I sold it in 2002 with 85,000 miles). I started with 15W-40, then used 20W-50 to quiet the lifters for a year or more until I could bring new lifters in my suitcase (hadn't heard of Auto Rx). Then I went to Delo 15W-40 and it ran fine. Never added a drop between 4,000 mile changes. Oil presures at 0 C were little lower (the guage almost pegged on 20W-50). This Jeep never saw normal driving. I normally flew to were I kept it and traveled to the mines and outback in it, mostly at 14,000 ft altitude.