I need some guidance. So I had a remanufactured motor put in my 98 xj. First 100 miles or so it’s running like a sewing machine. Runs amazing. My mistake was not doing the swap myself, found out the mechanic put synthetic in there and not break in oil.. Is it too late for me? Should I just drain out the synthetic and put in conventional or break-in oil? Or am I toast.

I've never even heard of break-in oil. I had to google it and it seems like a big hoax. I'd do very frequent/early oil changes. Like you could do one now then 500, 1000, so forth until you reach your normal interval.

Ok. So the fact he put synthetic I shouldn’t worry about and then just put Conventional, on my next oil change.

Correct, nothing wrong with synthetic, it's all made to be mixed.

Break in oil is important to use on engines with flat tappets, it has higher zinc content to help prevent excessive wear on the Cam lifters and bearings during break in, Diesel oil can also be used since it has high zinc content.

ok, Maybe i get some zddp with my next oil change? i used rotella in my old motor. but i think i want to stick with 10w-30 . is there a rotella 10-30? i know theres a 10-40.

As long as the cam was broken in properly you should be fine.
I like to used zddp additives or Royal Purple break in oil to break in older, flat tappet engines. Good for cam break-in and rings.

I use 5w-40 Rotella T6 after that.
Just my preference.

How can I tell if the cam is broken in or not? So far I’m 300 miles on the motor now and it’s running amazingly. Will the cam not being broken in give me any issues right now? I haven’t gone on the freeway much. Only once for like 2 miles and then remembered I read somewhere that I need to avoid freeways the first 1000 miles or so?

You wont know if the cam is broken in or not until after a lifter fails or doesnt fail. I would add a bottle of zinc additive as soon as you can. But whatever damage or lack of damage is already done.

The reason people say to not take it on the highway is that brand new engines dont like being held at a constant rpm. Harmonics non sense... Also, having the rpm go up and down will help break in the rings so thats where around town stuff would help. Down shifting greatly helps. It really doesnt take that long for them to break in.

You best resource for cam break-in is the manufacturer website.
Most tell you the first time you fire the motor you take it straight to 2000 RPM and keep it here, +/- 500 RPM for 20+ minutes.
Then you want to take it for a drive and do some vacuum pulls to seat the rings.

I recommend searching the FORUMS for engine break-in, but I don't think it will matter at this point.

Ok. Yeah I think it all worked out alright

Perfect Circle had a break-in procedure they recommended for new rings which was to idle the motor for 30 minutes, then take it on the road and crank it up to 4000 rpm in first gear, then let it decelerate in first gear, and repeat 10 times, then drive it normally. The theory was the acceleration phase heats up the rings and cylinder walls to scrape them in and the deceleration phase cools them and washes down the cylinder walls with oil. They also recommended the first oil change at 100 miles to remove any metal filings from the assembly and break-in. This was back in the 60's, so IDK what is common today.

Valvoline Max-Life 10W-30 contains a fair amount of zinc and is reasonable in price. I'm using Rotella T6 5W-40 Synthetic Diesel Oil in both of my 4.0's, but at $30/gal. it is a bit pricey. Advance Auto has it on sale occasionally for $20 so I stock up then. It only comes in gallons, not 5 qt bottles.

Almost all "synthetic" oils today are just ultra-refined base oil stock. I think Mobil 1 is the only really synthetic oil out there.

The zinc and phosphorous was recently regulated/capped in all API yada yada rated engine oil because of catalytic converter issues.
You can get Mobil 1 racing oil in 0W-30 (not for street use) that is their highest phosphorous and zinc, their high mileage and extended performance oils are for street use and are around the fed cap for those additives.
https://mobiloil.com/~/media/amer/us...pecs-guide.pdf

On the additive caps:
https://mobiloil.com/en/faq/ask-our-...rom-motor-oils

But adding the break in additive is best bet.

I use 10w-30 rotella t4 in my Cherokee and Comanche. Only local place I can find it is the advance auto. Ran break in additive with both motors for break in and ran it at 2000-2500 for cam break in. I don't know how much break in helps or hurts, but comp cams said to use break in oil for their cam, and the company I got the rebuild parts for my Cherokee from sent break in additive with the parts.

i just replaced the synthetic oil yesterday with T4 10-30 actually. drove to work and everything seems good this morning. hopefully everything works out ok for me since i never had the chance to use the break in oil... granted im probably at 350 miles total on the motor. hopefully everything will be ok