as i've done too much research on the matter, it's not the weight of the oil (which is meaningless) but the actual viscosity spec of the oil you should pay attention to. a 7wt oil from one company could actually be lighter than a 5wt oil from another. 
http://forums.mtbr.com/showthread.php?t=390797
for Marzocchi, i've gone with the original spec oil, which is Golden Spectro Cartridge Fork Fluid (125/150). i had to special order it from a motorcycle shop. not sure how you'd find fork oil at a car shop. :?
original thread i started regarding fork oil…
http://www.retrobike.co.uk/forum/viewtopic.php?t=108162
http://forums.mtbr.com/showthread.php?t=390797
for Marzocchi, i've gone with the original spec oil, which is Golden Spectro Cartridge Fork Fluid (125/150). i had to special order it from a motorcycle shop. not sure how you'd find fork oil at a car shop. :?
original thread i started regarding fork oil…
http://www.retrobike.co.uk/forum/viewtopic.php?t=108162
