I am by no means a jetting guru, but if I were you I would try adjusting the air screw as it has much more effect on idle than the needle position. It could be that it has been colder lately and you were border line lean before but now it is too lean. There also could be a slight viscosity difference with your new oil, or it could also be a combination of both things. The jetting 101 sticky in this section of the forum has some great information too. Good luck!
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