changed oil from 10w30 to 20w40, power loss?

So a dealer gave me 20w40 and said i would be fine.. well i always ran 10w30 and i told them this and they said nothing would get hurt by changing weights.. well after changing to 20w40 my bike just doesn't feel the same.. feels like it did when the bike was stock. could this be the cause of it feeling slower? should i switch back? and yes this is all i did was change the oil.

Unless you were running full synthetic and the dealer gave you a batch of used dino oil, I would guess it's all in your head.

Thicker oil does equate to less hp to the ground.. however its so small, I doubt you could feel it... If its bothering you... swap it out for some good clean Silkoleen or Mobil 1 MX4 of the same weight........