CR, here's what I have heard as the explanation: Synthetic
oil is just too good. That is, it protects the cylinder walls and
rings from abrading or "seating" into a proper fit. Remember,
in a rebuild, you score the walls of the cylinder (240 grit or so)
in a 60 degree pattern to hold oil and to allow the rings to
preferentially wear the cylinders for a better 'fit.' With synthetic
oil, the protection is just too good. The rings and cylinders never
wear-in properly.
As to why new engines use it, I can only assume that the
build tolerances on new engines are better than on our rebuilds.
That is, they don't have to wear their way to a good fit because
they already fit properly. Remember how long the break-in period
was on '60s vintage cars? Thousand-plus miles before you
could rev the engine or put any appreciable load on it. My new
BMW M3 had a sticker on the window recommending against
using it over 130mph for the first 500 miles! New cars just fit
together better.
Somewhere in this note is perhaps you answer.
BEst,
Rick
Charley & Peggy Robinson wrote:
> No one answered my question: If we shouldn't use synthetic oil for
> break in, which I've always thought made sense, why do some of the new
> cars come with it in the sump?
>
> Still wondering,
>
> CR
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