Mark:

Thank you so much for the reply.

Mineral oil may be an "old wives tale" but it is a very popular one and is still called for by Lycoming and Mattituck - the shop that did my work.

Yes, I know. But, Anatole France was right when he said that "if fifty million people say a stupid thing, it is still as stupid thing." This OWT comes from the 1920s and no amount of science seems to kill it. There is a lot of science which refutes it, but it's as though people are afraid of being called stupid if they disagree with it. I've seen very sharp engine builders, who KNOW this is silly, still recommend mineral oil just because they think others will think they're crazy if they don't! Go figure? I know quite a few others who no longer put mineral oil in their engines and they break in just fine. If your shop calls for it, USE IT. But, realize and accept that there may be no science behind their recommendation. There is nothing magic about it.

Break in can be also thought of as "wear in".  Modern multi oils work much better for reducing wear - maybe too well.

Multi-vis oils do not result in less wear than straight weight oils--unless they have anti-wear additives. I agree that avoiding those few oils with wear-reducing additives during break in is a good idea. (see recent Aviation Consumer article on this topic.) A recent automotive study actually showed that straight weight oils were better at reducing wear at high power settings. That's new data! They could not get the multi-vis oils to hold up during high power operation. (Auto engines operate a low power settings while aviation engines operate at high power settings by comparison.) For this reason, I have switched to straight-weight AD oil. I have been a multi-vis user for a very long time. My reasons were misplaced based on the science, so I changed with the new knowledge.

What you are really trying to accomplish is to smooth up the cylinder walls to a "just right" surface finish and fit.

Sort of. What you are trying to accomplish is the wearing of the RINGS' surfaces to fit against the still-honed cylinder walls. It is better if the cylinder wall does not wear very much against the rings. Some wear of both is realized.

A lot can go wrong with this process and the list of variables is huge.

Agreed.
 
A short break in may get the fit worn in past the point where a tight engine is producing excess heat but that may not be where oil consumption is minimal and compression is maximum.

That has not been our experience. As soon as the fit is right, the oil consumption is stable the compressions are as high as they are going to get . Are you suggesting that my engine at 5 hours with 79/80 compressions is going to get BETTER as time goes by? I have not seen that happen. After about 5 hours, we generally see a stable oil consumption for many hours until wear starts to increase oil consumption again. I've not yet seen an engine's oil consumption go down after the initial break in is complete. Of course, I break in an engine at about 85% power (maybe as high as 90%) and do it with cool CHT's, LOP so the mean pressure is high with the peak pressure out of the choke area. It can occur rapidly without heat damage. Maybe that aids my process over the people who toddle along at 65%-75% power.
 
Most people here leave the mineral oil in for the first 50 hours.
 
Again, Anatole France had it right. Most people run their engines at 25-50dF ROP, too. I prefer the science to the convention.
Mineral oil does not hold the particles in suspension. I want an oil to hold them in suspension and drain the particles out with the oil, not leave them in the engine.

YMMV, but most of all, the discussion is good. It makes us think about WHY we do the things we do. I'm trying to operate my engine based on the science.

Walter