My OH’s brother told me that recently. Apparently his mate who services fork claims that using a 15wt fork oil will ruin seals on the fork etc etc best to use 5wt oil. He’s a bit paranoid now as we recently used 15wt oil when servicing his Reba 🙂 Fork worked fine and did not explode after a short ride.

Is that claim a good candidate for bullshit of the week?

As long as the oil is full synth, I dont think it’ll trash the seals. Looking like 9/10 for BSOTW

If using it for damping then this will have no affect on the seals, if using it as lower fork / foam ring lube then this will have no affect on the seals. Using different weight oils may affect the ammount of lubrication the stantion gets in the lowers which may assist in early wear. The main thing that can affect the seals is the chemicals within the oil and oil height in the lowers.

Check the fork manufacturers recomendations.

BS

For here that probably barely registers on the BS-scale though 😉

15wt for lower lube and 5wt for damping in Rockshox forks

For large and/or aggressive all-trail riders heavier oil can really help reduce bobbing and brake dive on Motion Control without wacking up the compression. Does als slow rebound aswell, depends how you like it.

ruin seals on the fork

Nope, I think that’s a load of cobblers. I’ve used 15wt Stendec in magura, fox and RS forks for damping and lower leg lube. Not a single problem. 15wt seems to hang around longer too. 15wt was a bit much in my RS revs but I’m not a clyde.

As above, HS compression and HS rebound are normally non-adjustable but most forks have enough LS rebound and compression adjustment to cope with a wide range of oil viscosities.

Swapping the stock 5wt damping oil to 15wt transformed the performance of my previously dive-prone Magura laurins 🙂