I suspect the lighter weights are to make sure the oil gets readily into the axles in the roller lifters and the axles in the rockers. Since the OP is looking to use a straight synthetic, I think he will be in very good shape. The synthetic aspect of the oil will work better and it will actually be thinner in the temp ranges when the engine is cold and when still warming up, and so will get to those points better than a dino oil.
