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Break in oil - synthetics no good?

I recently had my engine in my '94 Miata replaced with a rebuilt one, and have strictly used Mobil One synthetic oil in it. The car is losing oil at a rate of 1 qt/500 miles, but NO drips or leaks, and I spoke with my mechanic about it. he said that I should not be using synthetic oil for the first 20,000 miles to let the piston rings seat themselves, and the synthetic oil was “too good” to let them do that. Is this nonsense, or is there something to it?

It sounds like nonsense to me. In terms of oil standards, there is no such thing as “too good.” It looks like you might have grounds to file a warranty claim … that is as long as you changed the oil as often as the owner’s manual requires. If you left the synthetic oil in longer than you would have left dinosaur oil in, you may have voided your warranty.

How many miles are on the new engine? How many miles did you go between oil changes?

It is worth noting that some vehicles (Corvette, BMW, Mercedes, Porsche) leave the factory with synthetic fill these days.

If dino only for 20K is a requirement, then your mechanic should have informed you of such.

Hmmm. Well, the dealer who replaced the engine has since gone out of business, so a warranty repair isn’t in the cards. The mechanic I spoke with works for a garage I trust and have done business with for a couple of years. I’m a “religious fundamentalist” when it comes to oil change intervals, and the car is treated like a jewel. But I would like to kniow where that oil is going.

Sounds like you need a compression and leakdown test, at least to see if either shows a problem.

The engine rebuilder decides what type of oil that should be used during the break-in period. Using a synthetic oil during the break-in period can arrest the break-in process. I would switch to regular oil and drive the vehicle under various conditions for the first thousand miles. After that, then switch to synthetic.


Where did that dealer buy the rebuilt engine? Surely the rebuilder has some kind of warranty. Did the dealership give you any warranty information?

I will ask again: how many miles are on the new engine? How many miles did you go between oil changes?

It couldn’t hurt to try! However, I am not sure I would switch back to synthetic.

OK. Here’s the bad news. The shop that I trust gave me pretty good evidence thet the dealer bought a junkyard engine which they (the dealer) then rebuilt. The engine was installed at 143k miles and the car now has 159k miles on it. I have replaced the oil at 3,000 mile intervals, and for the first 3,000 miles, drove the car very gingerly, not over 3500 rpm, but varied the engine speed, etc. Sice then, I have driven it normally. Oil consumption has remained steady at about 1 qt / 500 miles.

Use Mobil 1 10W30 for every oil change at 3k (except last change I let slip to 4, was a bad boy) I’m not original owner of my car, but I bought it at 108k. It’s at 148k and every change was with Mobil 1 and genuine Nissan oil filter (makes a big difference). Car runs like a top, and every time it goes in to my Mechanic’s garage, he always comments on how clean the engine is for it’s age.

Furthermore, I figure if Mobil 1 is good enough for race-car engines, it’s good enough for me.

When I did first buy the car though, I did notice it was loosing oil at quite an alarming rate; half the volume in the course of a week. I also discovered no drops on the pavement when parked. It took my mechanic a good three hours, a power-washing, and the car on the lift, in drive WITH the engine reving, to find the problem was coming from the front main seal. He said it was almost spraying out of the leak in the seal. Replaced the seal and it’s held every drop of oil since. I’d suggest that you have your mechanic do the same. Just to check.

This was a auto magazine topic when synthetics first became “all the rage” and the word was (either right or wrong) don’t use synthetic for break in period. Possibly some urban myth mixed with some truth (aren’t all myths?)

Who owned the dealership? Do the owners have any dealerships that are still in business? Even if they own non-Mazda dealerships, you might be able to get them to honor the warranty. it might be worth a shot.

I have to ask, if you are going to change the oil every 3,000 miles, why pay the added expense for synthetic oil? In my opinion, the advantages of synthetic oil only benefit engines that require it and those who want to extend their oil change intervals. Many people who use synthetic oil in cars that don’t require it do it so they can go longer between oil changes. That is why I thought you might have extended your oil change intervals.

I am beginning to come around on this belief that synthetic oil may have kept the rings from properly seating. If this happened, it is burning oil, and one quart per 500 miles is more than it should be burning at this point. Try conventional oil and try driving the car as if you are breaking it in for the next thousand miles and change it again. Don’t over-work the engine and don’t travel at sustained speeds for long periods of time while you are breaking it in. Don’t use the cruise control if you have it until it is broken in. You could also try getting some special break-in oil that some car companies use. Just make sure you don’t leave it in too long.

It might be too late to break this engine in properly, but it is worth a shot.

Many cars still recommend regular oil and some even have a break in oil from the factory, but not many any more. The synthetic is actually too fine a molecule size and too uniform and too slippery to allow the rings to take a natural set and wear pattern. I would not use synthetic until the engine has a couple of thousand miles on it with regular oil. then you can go to synthetic. My BMW and Corvettes and several other high performance cars COME with synthetic and require it all the time. The metallurgy of these internals is different and require no special breakin oil. Tolerances are tight and the engine will not burn oil, even without breaking oils. Some of the engines are actually broken in at the factory first.

Well it may be a little too late if the dealer is out of business, but if you followed the OEM owner’s manual for the car for oil choice and replacement, you did right unless you received special instructions when the engine was replaced. I would likely have used non-synthetic for the first oil.

However I think the oil choice issue is a mute issue.  Did you not get the car back from the dealer with oil in it, provided by the dealer?  Then the dealer made the choice and should have provided the correct oil.  After the first change synthetic would have not been a problem.

 I think I would try changing to non-synthetic for a few thousand miles and see what happens.