Well, the next question is what size do I need? I've noticed that grips come in quite a few different lengths: 115, 120, 122, 125, 130, 135mm..

 

I've just been measuring and I think i need 120mm, do you recall which size you used?

 

Just measured my F4i and the 120mm length open end grips are correct.

 

Check out Renthal and Driven. They both make quality gel grips.
Bar-end weights can help dampen vibration, as well.
Consider gloves with gel-pads, thrown into the mix.
Speaking of gloves...do yours fit properly? If they are too snug
across the palm, they can be cutting off circulation.
Consider a CrampBuster (or some other throttle-assist),
they keep you from gripping too tight (especially on longer distances).

Ern