Is this wire something you already have on hand? If so I say give it a try but I personally think 18 is too light. I would use at least 14 for wind and birds. I only use 12 AWG solid naked copper myself for a wire antenna. It is very strong against heavy winds, is somewhat flexible, and very workable with deployment. Very tough stuff, it develops a patina coat from the salty, moist air here, but underneath that coat remains the full strength and signal surface of that heavier gauge.
As Ridgescan said, the smaller stuff can stretch if the winds are high enough, or well fed birds land on it. If it stretches, it changes its length and resonate frequency to a lower channel. Worst case is that it would break.
But, you can certainly try it. Make sure it gets installed in such a way that it's not under too much strain, like being tied off to a tree branch that blows around a lot. You can also try using bungee cord/shock cords on the ends as a way to let it move without breaking.
Another consideration is whether or not the feed point will be supported and what coaxial cable you use. The weight of a long run of RG-8 can cause a lot of stress. If it isn't supported, you'd probably want to use something light like RG-6 for this application.
If you can, run the end of your antenna tie rope through a pulley to a weight. The weight should be enough to keep the wire and rope straight, but allow it to sag if it gets icy, or sway in the wind if it needs to. I've done this with both of my end fed antennas. I'll go out and snap a picture of the pulley setup and add it later. Use a couple cable clamp u bolts and a wire guide to run the wire up through the eyebolt on your mount as seen here: Best way to hang the W4KGH matchbox | W4KGH Amateur Radio Antennas & Accessories This keeps the strain off the post where the wire connects.
It should work for an 11 meter dipole. If it's insulated, that would be a plus, the insulation would add to the overall strength and weight. As the others said, it might be center heavy, because of the coax, but that wouldn't affect the reception much.