Either conventional or synthetic oil will allow break in if you employ the right method...
I was stationed in Japan years 1979 to 1981 as a member of the USAF
and I witnessed Honda build a motorcycle in 12 minutes and then run at
red line in neutral... I asked about the "break it in gently" in the
owners manual and the replay was the "break it in gently" are written
by their corporate lawyers not by their engineers... if you follow
their engine rebuild steps in their official shop manual there are no
reference to any "break it in gently" warnings...
Your not thrashing your engine... you're doing what your engine requires... The purpose of high
rpms is initial seating of the piston rings to the cylinder walls. The run is conducted at full power
because that is where greatest B.M.E.P. (Brake Mean Effective Pressure) occurs and a high
B.M.E.P. is necessary for good piston ring break-in.
The cylinder walls of a new engine are not mirror smooth as one might imagine. A special hone is
used to put a diamond like pattern of "scratches" over the entire area of the cylinder wall. The
cross hatch treatment of the cylinder walls plays an important role in proper break-in of piston
rings to cylinder walls. I mentioned that B.M.E.P. was necessary to the "break-in" process. Here is
how it works.
How do you know if break is done??? take a compression test... if your
engine shows factory compression then break in is complete... if your
engine shows less than factory compression then more break in is
require... more B.M.E.P. (Brake Mean Effective Pressure)