Notchy shifting after oil change

My 01 yz426 has lived its entire life with Mobil1 15w-50 red cap. The guy I bought it from used it exclusively since new and gave me an unopened 5 qt jug when I bought the bike from him. I used up all the red cap oil and when I just did my oil change yesterday I used the 15w-50 currently available at WalMart (with a regular silver cap).

Since that oil change, my shifting is very notching. It was never buttery smooth, but it's very rough and notchy now. Shifting with or without the clutch doesn't make much of a difference.

I don't want to start another oil debate, but is there anything wrong with the current 15w-50 from Mobil1 or any reason it would cause notchy shifting?

I'm considering the dino Rotella 15w-40 because it's dirt cheap and I've heard good things about it. I've been changing the oil every other ride, so I'm not too concerned about it breaking down sooner than synthetic.

I have a friend who can get me Amsoil MCF 10w-40, so I may try that too (despite the high price tag) and see what works best for my poor gearbox .

I am using the Amsoil oil in my 00 426. I noticed smoother shifting using that oil as soon I switched to it. I was using Belray Thumper prior to the Amsoil. I have never used the Mobil, so I have no way to compare it. I am happy with the Amsoil, however.

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