10W-40 has too much VI improvers and thats no good, especially in motorcycle applications where the oil is subject to heavy shear. Too much VI improvers also creates problems with gumming up the piston rings. There are numerous articles on this and i dont know why bike manufacturers still recomend this grade. 10W-30 is a better choice here. I personally use shell 15W-40 Rotella T which is a heavy duty diesel engine oil also approved for gas engines. It is extra shear stabil and VI improvers are kept in ratio so it is good for bike engines. Many people use it in motorcycles and have great results.
If you are going to use a 10W40 grade, make sure it is synthetic oil, as you get away from the VI improvers.
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