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Hey guys, I'm about to swap the stock fork springs for some heavier ones (.42 if I remember right). I was wondering if I should go with heavier weight oil, like 10w. Anybody got some advice?

What bike, and which 10W oil did you have in mind?

It's an '03 kdx 220, and I'm not sure what brand of oil - just a quality fork oil of some sort. I would welcome some suggestions.

PJ 1, maxima are all good brand. i would go with 15wt to slow down the rebound as there is no rebound adjust on the stock fork.

I would try Maxima 5W fork oil or Belray 5W which is a little heavier.
I used Belray in mine and thought it was a little heavy, but I had .38 springs.

Check what your manual says, it's surprising

I'm subscribed to an online manuel as well as having the factory hard copy manuel in my hand. I've checked both and neither one specifically addresses what weight oil should be used with stiffer springs. Maybe it doesn't matter. The factory manuel does say something interesting. It says for a quicker rebound, we should use more oil, and for a slower rebound, we should use less. That seems backwards to me, but what do I know? Can anyone shed some light on why that is true? As far as oil weight goes, the factory manuel says either kayaba 1 or just regular old sae 5w-20. The online manuel says 5w. I've yet to find a satisfactory answer for what is best if going to a stiffer spring. I guess it'll have to be trial and error.

I was surprised to read that the manual says 5w-20. My KX manuals says the same. I have XR manuals that read ATF for fork oil.

Go for 5 weight.

When it mentions oil amount and rebound, that would be affected by the air spring set by the oil level.