I just looked in the engine owner's manual that came with the 23 Vanny kit I just got for my dad's cart... Briggs primary recommendation is 5W-30 Synthetic, dino oil only if synthetic is not available.. No mention of not using synthetic for break-in.. I've been running Mobil 1 5W-30 in my 23 Vanny since day one, with no break-in issues at all..