One leaking fork - advice please - fix one or both?

[map]

Just a quick request for advice.
My lad's bike, Honda XL125V, has one leaking fork.
The dust seal looks like it's disintegrating.
The other side looks fine and no leak.

So, based on the fact I've been told these things are best replaced with OEM kit rather than 3rd party...

...do I just replace the one side?

If it's both sides than finance commitments atm means no OEM kit.

Downside is I'm not sure what weight fork oil would have been used. It appears 10W is the standard but as bike not had from new not sure if anyone would have had a play (e.g. tried heavier to firm up suspension).

Words of wisdom from those who have gone before and all that
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...and the whirlwind is in the thorn trees, it's hard for thee to kick against the pricks...
Gibbs, what did Duckie look like when he was younger?

[carvell]

When I changed my fork oil from the factory standard 10W to another 10W, it really improved the front end. I assume this is because the oil had been in there such a long time that it had deteriorated and was performing badly.

Based on that, I wouldn't really want one fork to perform badly and another well, so I'd definitely do both.
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Yamaha TDM 850

[P.]

I'd lift the dust seal on the good one, make sure its all good underneath.

You can change the oil in both, on the good one just loosen the top bolt on the fork a little so its easy to remove when changing the oil.

Change both to 10w and just replace the knackered seal.

My Vara has been leaking for ~3 months now. I just can't be bothered to replace currently until I have the funds to get them straightened..

[CaNsA]

Sods law dictates that the day after you fix the leaky fork the other will pop.

Replace both with fresh seals and check the bushes while you are at it.

[Islander]

I'd always do both sides - they're going to be worn to more or less the same extent.