Most factory and the budget aftermarket synthetic stocks weigh pretty much the same as wood and are often heavier. There are a few exceptions, but even then the weight difference is insignificant. Most factory wood and factory synthetics will weigh 30-36 oz depending on who made them. Ruger and Remington factory synthetics tend to be on the lower end at about 30 oz. About the same as a factory wood Remington MT rifle or Winchester Featherweight stock.
The standard fill McMillans and other high end stocks made with fiberglass will be in the same 30-36 oz range.
The B&C Medalist and HS-Precision synthetics weigh 36-40 oz
The Hogue synthetics weigh either 40 oz for the versions with pillars, the bedding block versions weigh 48 oz.
Laminated wood is generally in the 36-48 oz range.
You don't get a truly light stock until you get into the high end stocks made with Kevlar. A McMillan Edge, Brown Precision or MPI lightweight stock will be in the 16-24 oz range depending.
Synthetics don't make the rifle more accurate. They do prevent the point of impact from shifting due to environmental changes which will make any wood stock expand or contract. Even the cheapest versions are tougher and less likely to break if abused.
Is there anything you can use (spray, type of wax) on wood stocks that help them battle the elements better?
There are a lot of ways to help prevent moisture from entering, but that doesn't address the real problem. When wood is dried in the kiln they have to leave some moisture in the wood. Too dry and it will break easily. Even if you completely seal the stock it is the water inside that expands and contracts as temperature, altitude and humidity change.