They may have the same engine block, but the tolerance from the piston ring to cylinder could be different, and bearing and oil passage clearance will be different. Toyota recommend 5W30 for your ES and my 2006 RX, but 5w20 is also acceptable. You may lower the weight, but not for higher weight, at least not for prolonged period. Why want to go against manufacture's recommendation? Another poster above run 40 weight oil, but I think that's foolhardy.Nope..They are the same.And different pistons wouldn't have an effect on what oil weight to use. Bearing clearances would have a big impact...but not pistons or piston rings.