There have been several post about what oil to use to break in an engine. Kawasaki and Briggs and Stratton often recommend strait 30wt. Some have said that you should use synthetic at break in. Here is what I found that NASA has said about engine break in: I am a NASA Engineer at Marshal Space Flight Center in Huntsville, Alabama. My field of work is Tribology which is the study of friction, wear, and lubrication. In our spare time, our group collects and tests different motor oils using the Shell Four Ball tester. This tester tests the extreme pressure properties of oils. These areas in a motor are cam to lifter contacts, valve stem to guild, and piston skirts. Over the years we have found synthetic bases oils to out perform mineral based oils by a large margin. Do not run synthetics in a new rebuild. A new engine needs the added friction allowed by mineral oils to set the rings properly. Chevy found this out on the corvette. These cars came from the factory with Mobil 1 and owners brought them back because of smoking and oil consumption (rings did not seat). Run a good single weight oil for the break in period. We found Havaline 30 to be a good mineral oil - in fact we use it for our standard. Phil Hall, NASA OK, what I got from those excerpts is Synthetic is superior but not during break in. He also goes on to say multi-viscosity dino oil breaks down faster because of additives (faster than 30wt). Adding thickeners like polymer chains in multi-visosity dino oil (which may make them break down faster)are not necessary in multi-viscosity Synthetic oils.