Ferrofluid tweeter

I recently purchased a pair of B&W P5 speakers that were made in 1997. After making sure all the old ferrofluid was removed entirely, it was time to add the magic. While adding the ferrofluid, I purposely put 60ul into on tweeter and 100ul into the other. One thing for sure, it made a huge difference in voice clarity, no more muffled sounds coming from the tweeters.

There is a slight difference between the tweeters because of the different amounts of ferrofluid used. The tweeter with 60ul is a little brighter than the 100ul, but can’t decide which one I like better. Once I decide which one to go with, I’ll even the two tweeters so they will match in sound.

Has anyone used less than the required amount of ferrofluid in a tweeter, and would there be any harm in doing so?

The tweeters I have are 1 inch and require 100ul.

As far as hearing goes....Tweeters only make a ssssssssss sound, they don't handle freq's that shape mid range vocals, they do have to move very fast and handle a lot of power so tweeters are the most rugged driver in a speaker, I would put the fluid back in that is required by the manufacturer so they don't overheat during long loud periods of listening. 
I also recommend that you use the proper amount of ferrofluid.  Tweeters are designed with or without it for a reason.
Ferrofluid has consequences of good and bad, that's why its important to use as designed.
Ferrofluid does help absorb heat, so it does help with power handling. Ferrofluid also adds resistance to coil movement, so it changes QMS.  Too high or too low and you will be dealing with a peak or dip. So you can see here why one would be brighter.
Overall, If you don't overpower your tweeter, It should live, but my experience is that manufacturers use ferrofluid for a reason.
Focal uses it when necessary.  Seas doesn't use it (at least that I've seen) If your careful, you should be fine.
Pretty much all dome tweeters use ferrofluid. In addition to above points by timlub, it also helps align the voice coil. Small voice coils in a tweeter are susceptible to rocking motion.

Ferrofluid is a cheap solution to building a tweeter that will perform well for about 5 to 10 years. Usually performance declines audibly after 5 years and sometimes as little as 2 years. Replacing the ferrofluid is a good idea on 10 year old tweeters.

ATC now make all their transducers in house and their soft dome tweeter is one of very few that do not have ferrofluid. In order to overcome alignment or rocking issues they use a double spider to stabilize the voice coil and to overcome heat build up the manufacturing tolerances are very tight - costly to make.
My Hyperion HPS-938 speakers use ferrofluid for 6.5" midrange driver suspension (no spiderweb) and most likely in 1" short horn dome tweeter. They are at least 5 years old but I cannot hear any sound deterioration (I think). Hyperion Sound doesn’t exist anymore and I have no way of knowing what type or amount of ferrofluid is needed. Is replacement of ferrofluid a standard service done by the music store, speaker technician etc? What are the symptoms of dry ferrofluid?

I too need to clean out the old ferrofluid and put in new.  Unfortunately I'm not super handy...if you are willing to share tips on how you took out the tweeter and anything else I need to know for this operation, I'd super appreciate it!
When you clean out a tweeter you need to remove the voice coil and diaphragm assembly carefully, then remove the old ferrofluid using absorbent material.  Coffee filters work well because they don't have dust like paper towels can.  Use  strips of filter paper that you insert into the air gap and soak up the fluid.  Use enough so that when they start to come out sort of clean you know you have gotten most of it.  Then soak a strip in acetone or heptane and wipe the gap well.  Also use heptane or acetone to wipe down the coil itself to get the residue of the old fluid off.  Once this is done, allow it to air dry for a few minutes.  You can then put the new fluid in and re-insert the coil assembly (or put in the new one).  When properly filled, the fluid should bulge slightly over the top of the gap.  Under-filling is a common cause of poor performance in fluid filled speakers.  Hope this helps!    -Vanessa
You should always fill the air gap with the recommended amount of ferrofluid.  Under-filling is a leading cause of poor performance in a driver.  Under-filling can actually cause problems related to overheating - the fluid will be stressed and thicken prematurely, leading to loss of signal clarity and shortening the usable life of the driver.  Good luck!  - Vanessa