Depends upon the tolerances of your rebuild.
I'm not up on synthetics, but with the tolerances possible now during modern manufacturing most new car manufacturers are recommending 5w30.
15 is pretty heavy especially for cooler climates. Your break in oil should certainly be less viscous.
Some guys used to build their cars "loose" thinking it would give them a few more rpm's or horsepower.
I personally never belonged to that camp. I'm all for precision and tight clean fits.
I'm not up on synthetics, but with the tolerances possible now during modern manufacturing most new car manufacturers are recommending 5w30.
15 is pretty heavy especially for cooler climates. Your break in oil should certainly be less viscous.
Some guys used to build their cars "loose" thinking it would give them a few more rpm's or horsepower.
I personally never belonged to that camp. I'm all for precision and tight clean fits.