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Thinner oils allow more spring temp too

I suspect (no actual data) that thinner oil may have lower surface tension, and form smaller droplets in the cover, get around better, and cover more area before draining.
I'm surprise that I haven't seen more remarks about something already in use elsewhere: a lower spring cup on the spring OD, normally used to prevent chafe and locate the spring (rather than a seat on the guide OD) with a taller lip than normal. The cup retains some oil, and when the spring is really moving, it acts like a bird in a birdbath - it splashes the oil out of the cup very nicely.

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