3-in-1 is often SAE 20, as is ATF.
If you google around on the web you'll find many a row had broken out on this subject. It's all much of a muchness - the important thing is to ensure there is damping to prevent needle flutter, but it's not so much that the piston is really 'heavy' to lift.
The important thing to remember is that piston damping is controlled by the size of the billets on the damper stalk, NOT the viscosity of the oil. As such, the dampers will have an AUD number on the screw cap - so make sure they're the same. There wouldn't be an exactly specified damper if it was intended to control the damping with the oil grade, so don't mix and match and keep everything nice and consistent.
As far as the level of damping is concerned, remove the air filter elbows and feel how much resistance there is with your finger when lifting the piston (engine off of course!). It should be about the same resistance as pushing an empty mug along a table. Any less (ie, barely any discernible damping) may be a bit rattly, and any more (ie, you really have to force it) will artificially restrict your engine's responsiveness, and potentially cause lean running overall if doing a lot of throttle-on-throttle-off driving (ie piston is too slow to reach necessary height before dropping again when you back off the gas).
The acid test is to pull them both right up to the top of their travel and let them go at exactly the same time. Listen for the pistons hitting the bridge at exactly the same time. If they don't you need to take the dashpot off and thoroughly clean the piston and chamber with carb cleaner. The fall time is governed by the precision fit of the pistons displacing air at the same rate. So cleanliness is what makes them work properly, not introducing variables like different springs and damper oils.
Good luck!
Michael
