Oil level should be between 45-55mm from top of stanchion. If you’re rebound damping adjustment doesn’t appear to be doing anything, it might be because you used oil that isn’t thick enough.
The oil ratings written on bottles are fairly meaningless, the only thickness rating worth a damn for our purposes is centistokes, an international standard. This site has the viscosity in centistokes of various brand fork oils, and also illustrates why the manufacturers’ published weights are useless:
http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid
Marzocchi use a 26.1 cSt oil, but remember the ideal oil weight depends on how much air preload or which rate spring/s you use, which of course depends on your weight.
The easiest to source/cheapest synthetic fork oil is probably Castrol Fully Synthetic Fork Oil. Their 5w is 28 cSt, with the 10w being 42 cSt. They can be mixed in various ratios to achieve anything in between.
I’m a large chap (200lbs+) and have converted my Pros to coil only (heavier guys need so much air preload that the forks lock out before reaching full travel). I’m running coils totalling 8 kg/cm at 120mm travel which requires 38 cSt oil to achieve a nice rebound rate with adjustment knob in middle position (i.e. 4 turns from fully open/closed). I mixed 30 ml/cc of 5w with 105 ml/cc of 10w to achieve approximately a 38 cSt mix.
