I'm not sure if you're still waiting for a reply, but as it's been over a month and no one has responded I'll throw in my two-bits. As my 122S with twin SUs got older, I used to check the oil in the damping cylinders at every oil change. It was rare it needed topping up, maybe every 3rd-4th oil change. I'm sure it could have gone much longer before being at the minimum level. If you're not familiar with it and as you likely go a long time between oil changes then maybe check it every six months until you learn to trust it and can go for an annual check as part of spring tune-up. Roughly every 2nd year I used to drain it and clean-it up before re-filling
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now