The best is flax oil. The next best is soybean oil. The third best is liquid canola (not hydrogenated Crisco).
This is because of where those oils are listed on iodine index; which is a measure of how much an oil will polymerize. Polymerization is when oil turns into plastic and is the actual chemical process responsible for "seasoning".
Here's a whole site dedicated to this topic with more detail:
Chemistry of Cast Iron Seasoning: A Science-Based How-To
How might I detect varnish?
No utensil intended for real use (other than perhaps as a salad scoop or bowl) should be varnished.
It may be difficult to distinguish a varnish from an polished and oiled surface, but look for a clear shiny type of appearance where the grain of the wood is completely masked.
Are there other steps to seasoning this type of spoon?
Just rub it with mineral oil, as described below, if it is dull and untreated. If it is already seasoned, there is no need.
You may want to repeat the process two or three times, for a brand new utensil.
[I]s there anything to watch out for when washing/drying?
The main thing, as you have already mentioned, is to not use the dishwasher. Don't soak it in water. When you wash it, give it a quick towel dry and then let it air dry.
You don't want water soaking into the grain, which will help raise the grain and may eventually lead to splitting or cracking (at which time the utensil should be discarded).
Is there an upper temperature limit or hot surface type that should be avoided in cooking?
I don't know a specific one; use your good judgement. Wooden tools are often used in scraping up the fond from a pan after deglazing, which is probably the hottest treatment they would normally be exposed to.
As you happen to have a ladle, you are unlikely to be using it on the bottom of scorching hot pans in any case.
Wood is actually quite fire resistant, and while one may show a few scorch marks at the tip from a very, very hot pan, in general, routine use is not going to harm it.
Should it be maintained by say rubbing it down with some type of oil once in a while?
Yes, occasionally you will want to rub it with a little food grade mineral oil (which resists rancidity). This will help prevent liquid from soaking into the spoon.
When you see signs that the last treatment is wearing off, wipe it down with just a bit (maybe a half teaspoon or even less for the whole ladle), let it sit an hour or tow, then rub off the excess.
This will help you get a longer life from the ladle without splitting or cracking.
See also:
Best Answer
No need for elaborate application techniques (brushes, spray, etc.).
When I worked in an old style restaurant, the Chef had me:
At home, you could use a painters wire brush, or if you are really motivated: wire brush wheel on an electric drill. Or a stainless steel pot scrubbing brush. The one I used in the restaurant was similar to this:
The same brush was also used to clean the grill grate.
You can use any rag you want for the oil application. Strong paper towels would even work.
For a home block, I'd guess you'd have 1 tbps absorbed by the block (that's a good thing). You'd probably loose 1 tbsp in the towel--the cost of doing it right.
We just used very hot water on a towel to clean. Soap was forbidden. If it was soiled and permeated by something nasty, we would do the entire wire brush cycle again.
** Cleaning with the block with brush: I did it with a dry block. Some of the others would oil the block first, then liberally salt for the abrasive property, then wire brush it. My way gave similar results, but was easier.