The "How do you start your engine" thread prompted me to post this. I've been doing a lot of reading about motor oil in old threads here and all over the net until my head is spinning. When I got my Jeep I removed the oil pan and found a layer of sludge on the bottom. I cleaned it out and then ran some Seafoam in the new oil. I used Castrol 10W30 because I was told it had lots of detergent. I changed the oil several times since then, always with 10W30. The Jeep always started great-- choke out, 2 squirts and fired up. The oil pressure was around 20+ at idle and 30+ at speed. I could hear some clacking when winding out 2nd and shifting to 3rd. Then, because of what I've read about zinc, flat tappet engines, etc, etc, etc, I decided to try some Rotella T 15W40. Now the Jeep is a little harder to start as if it's trying to overcome the resistance of the thicker oil--doesn't always start on the first try and have to crank it a little. Even after it's warmed up, it seems to be battling the thick oil, idling roughly enough to make me increase the idle speed on the carb. The oil pressure is now up to 30 at idle and 40+ at speed. The clacking is greatly reduced or gone and the leaks are fewer. Did I do the right thing in switching to the 15W40? If it matters, the weather here has been in the upper 60's low 70's. It will stay over 100 all summer. Which weight oil would you recommend?