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Funny you say that, because - while I don't have 40 years like you, I have over 15 with high performance and race engine builds, both boosted and naturally aspirated from 1 cylinder to 12., both gas and diesels. My findings between the two always showed - no difference in bearing wear, no abnormal cylinder wear, scuffing, or broken rings on the hard break in engines, and as a whole generally higher compression between similar engines broken in both ways. Now, when I say this, keep in mind that all of the hard break ins were done in a very methodical and controlled fashion, often times on a dyno or other controlled environment. Occasionally on a track. I am not talking about Joe Q. Public taking a new engine and simply thrashing it everywhere all of the time. That isn't a hard break in, that's simply abuse.
Just for giggles, I can point to my Jeep as an example. I pulled the factory engine 142,000 miles ago due to piston slap, a common issue with the Jeep 4.0L in the early days. No high dollar parts to speak of, just your basic refresh - polished main and rod journals, .010 overbore and hone, new pistons rings, RV cam, yadda yadda. All done by me. It had a hard break in done to it. The motor has been apart since then for various maintenance items, rear main seal for example. Bearings are no less typical than any engine with the miles it has on it. Cylinders still show cross hatching, and a compression test done last year showed all 6 cylinders to be within 3psi of one another and all on the high end of the scale for what this engine should make.
I am more than happy to continue this in a PM, and I mean that. I like discussing alternate points of views. As I said before, this subject is often controversial and tends to cause, at best, debates.
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