http://www.homedepot.com/prel80/HDUS....jsp?pn=111310
It is a type of oil, you could also use sewing machine oil in place of 3 in one oil.
I made my own valve oil for a year or so, it makes the fastest valve oil I have ever seen but it does evaporate. I had to oil my valves everyday!
Also, if you forget your homemade oil on a gig and have to mix with other valve oil you could have problems.