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More properly called piston dampers, many back in my Amazon days just called these dampers and the slides were damping cylinders. I never used engine oil, always ATF, but it may not matter a whole lot.
Doing a little reading, almost any kind of oil will work and recommendations are all over the map. You could probably use Crisco oil from the supermarket and not notice a difference, but it would soon go rancid.
I just refreshed my memory that there was a special SU Damping Oil known as SU20. It was a type of dashpot oil and quite thin. The genuine SU brand appears to still be available online. I recognize the bottle, now recalling I started with a small bottle from a local dealer service, probably a partial bottle the mechanic sold me, but ATF was cheaper and easier to come by so switched. There seem to be various opinions of an SAE multiweight with equivalent viscosity characteristics. On place says OW100. Another says 0W50 dissappears more more slowly.
Gerhardt's Volvo Problem Solver recommends using ATF for the SUs, as do many, and filling to about 1/4" from the top (using the plunger as a dipstick). That should last quite a while. I rarely needed to top mine up to ensure the piston was always immersed in oil, just checked it at every oil change. It also mentions keeping the slide area clean. Occasionally I would flush mine, wipe it out carefully and fill with fresh ATF. I would have used a bottle brush to get any residue loose from the bottom. Knowing me, I probably tried using rock wool to polish the walls.
If other fluids are used, like motor oil, then I would go for a viscosity similar to ATF to achieve a similar damping affect for acceleration and economy of fuel use. ATF is thinner and higher overall viscosity than 10W30, apparently equivalent to about 5W20, some say 0W20. Detergents and additives in motor oil will mostly just be along for the ride in that application.
Consistent viscosity over a wide range of temperatures is what's needed for uniform damping, but at the same time having a slightly variable viscosity affecting damping under very hot or cold conditions may benefit running, such as at cold start. I'm not sure which of ATF or 5W20 would be better in that regard, or if the difference would be measurable, but whatever the Swedes recommend is what I would do.
Hopefully Planetman Eric follows the 240 area of this forum and will chime in with his sage advice.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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