I havent tried izo but I'm guessing it's made for 3x3s which means min 35k max 45k. You can use the approximations used by lubix, its more or less accurate. I find they are a bit too generous though, so get more than the exact amount the lubix approximations guide you to get. According to lubix 10cc = 60 cubes.
I bought a 4cc of lubix quite a long time ago (before christmas thats for sure) and I just finished it a week or two ago.
Traxxas sells diff oils at 10, 30 and 50k. I guess 1 = 10k, 3 = 30k and 5 = 50k, 2/4 being a mixture of 10/30 and 30/50. 50k is good for cores, maybe a bit on the heavy side for pieces. 30k is great on pieces, maybe a bit too quick. 10k I guess would be way too fast and runny for 3x3 meaning that it will end up in a puzzle thats hard to control and in the lube wearing out too quickly. 10-30k doesnt work too well with cores, 30k is okay I guess ( I have that and have used it). 1-3 would be for bigger puzzles and 3-5 for smaller puzzles and cores.
Refer to the customer reviews:
http://thecubicle.us/assorted-regular-silicone-lube-p-61.html