I would be very skeptial of a engine builder wanting you to use a multi-grade oil to break a new engine in espectially a solid lifter 6 . That said my reasoning is that you want all the heavy shearing action and cushing the heaver oil provides with it's special addtives ash,phospates,zinks etc. plus less chance for the 20,30 wt breaking down under pressure during the breakin. I have a friend who uses syn oil on his vehicles Cars Truck pickup and loves it. He told me the seceret was to break them in on regular oil first them switch to syn. On street engines they require a different method of breakin compared to a race engine. Remember race engins are built for one thing Testies to the wall where the street is built to last over the long run.
62Blue
Don
