I had a reman 4.0 in my 2000 Cherokee.Started with mineral oil,then to 5w20 for 500miles,then 10w30 dino for about 1000miles and finally Mobil1 10w30 full synthetic. It broke nice.
Fresh engine- what oil?
- Thread starter Bonnewagon
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Yes, I always did that too. Tricky with an unknown carb or distributor. That is why I would set the timing by cranking the engine minus spark plugs. Best way ever. Then I would use a known good carb set for a very fast idle. Once I get the cam sensor dialed in the timing and injection should be just as before.
Exactly what I did with my rebuilt 301. After 1000 miles it has seen only Mobil1 and is still tight and clean. But no Cats were involved.
My head hurts. Not enough zinc- too much zinc- good for cams- bad for cats- YIKES!!! I'm leaning towards a mineral oil with a good amount of zinc but not too much and with a high “Load Carrying Capability/Film Strength/Shear Resistance” rating. It also has to be readily available and not stupid expensive. I see 5W30 Chevron Supreme as having 1018 ppm zinc, 728 ppm phosphorous, and 161 ppm molybdenum. All while having a 110,011 psi wear protection rating. Being winter that will flow well. In the spring I can change to 10W40.
I don’t think the amount of zddp is going to hurt the cats in the short amount of run time you are talking. Catalytic converters have been used since 1975 and oil had tons of stuff in it. I’ve run 15/40 diesel oil in my 06 suburban for 1000’s of miles at a time when I had some left from the race car.
Assuming the builder used high zinc assembly lube (which all good builders do), we use a cheap 30 wt dino oil. Run for about half an hour for break in, fluid checks and leak inspections. Dump the oil and change the filter, get rid of the assembly lube and crud. Use high zinc dino oil. We like Delvac 10-40. Break in 500 miles varying rpms and load but no wot banzi runs. Been doing that for everything from stock to 9 second cars...
Yes, I have been at this a while. Years ago I would have used the cheapest oil I could find and run it first for break-in. Then dump it and change to a good 10W40. But even the cheapest oil had plenty of zinc and there were no Cats to worry about. This is the first fresh engine I have had that had Cats in place. Talk about a juggling act. Flat tappet cam needs zinc but cats can't have it. The last fresh engine I had done is the 301 in my Bonnewagon. No cats. After break-in on Cam 2 mineral oil I switched to Mobile 1 15W50 which still had a good shot of zinc. It has had that for 25 years and it is still tight. I agree a zinc rich oil for the initial run probably won't hurt the Cats but the first 1000 miles after that is what I am worried about.
Well, the easiest solution to not mess up the cats is don't hook them up. Run a straight pipe for the first thousand miles! 
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