I might be on crack, but here's my take on it (having owned&tinkered with many marzocchis):
Using springs for pre-load generally creates a more linear feeling pre-load than air, which is by nature more progressive. Especially at lower psi (sub-20 most likely for you). Of course this depends on the volume of the air chamber- the smaller, the quicker it's going to ramp up. Add enough oil to your fork and you'll never bottom it out, regardless of any preload, springs, or rider weight. however in the first inch of travel (sag), 10psi of air preload is not really compressed enough to afffect the fork. Past that it starts ramping up quickly.
using a coil spring for pre-load will cause the the fork to ride higher in travel, possibly reduce small bump sensitivity somewhat, but will maintain a more linear travel. This also means less resistance to bottoming out compared to using air for pre-load, or adding oil.
Basically, if you want to prevent bottoming out, add more air pre-load, or more oil. If you need more than 15psi of air pre-load, it's time to raise your oil level.
If you don't bottom out, but feel like the fork has too much sag or rides too low in it's travel, then heavier springs might be the ticket (presuming you've tried adding oil and up to 15psi first)
Either way, I would only go to heavier springs if you're still bottoming out the fork after adding 15psi and verifying the oil level is not lower than marzocchis reccomendations.
Soupboy said:
but I went through the same thing with my 03 Z1FRQR20. Since I have the time and would do the oil change anyway I figure I should get the proper springs in there.
Plus, doesn't running air preload on the high side negatively effect small bump compliance? Maybe not.
I'd rather have the right springs than run a bunch of preload. Then again, my Z1 didn't have this feature so I'm not that versed in the proper set up.
Sean