I used to add a little tin to WWs for several reasons, but now that pure tin and bar solder is expensive and hard-to-find cheap, I save my tin for special alloys and cast most of my bullets with pure WWs. All my moulds fill out just fine as-is. For troublesome moulds, use a quick light spray of Frankford Arsenal Drop Out for perfectly cast bullets right off the bat.
I have found that WWs are fine for bullets in most revolver and pistol loads where the muzzle velocities are <1300fps. With low-pressure cast bullet loads in some rifles, I will sometimes use WWs up to 1600fps, but prefer to add some tin at that point. A good high velocity bullet lube helps prevent leading more that adding tin, IMO.
Some folks add a little linotype to their WW alloy for better castibility and to add hardness. I haven't tried it because I don't have access to any of that stuff .
WW alloy contains about 3% Antimony (Sb), which is okay for 99.99% of cast bullets. They used to contain, about 15 or 20 years ago, 9% Sb. That was even better. Why this was changed - I haven't a clue. Sh*t happens. As mentioned, by xtm, linotype is good stuff to add to bullet alloy if you want filled out bullets with a slightly higher Brinell Hardness Number (BHN). Fortunately, for me, I managed to latch on to about 200 pounds of the stuff twenty years ago for next to nothing. That is the source of the Tin (Sn) when I want slightly harder bullets for auto-loading cartridges.
Anywho, WW gives you about 10 - 12 BHN. For auto-loading cartridges, I prefer a slightly harder alloy - 15 to 16 BHN - for better performance. What I mean by "better performance" is movement from magazine to chamber during the loading cycle. There is less chance that the bullet will deform and hang up. Yeah, I know - like about a bazillion to one that that'll happen. Like I said, sh*t happens.
If you're worried about leading - don't. The primary cause of leading with cast bullets is improper sizing of the bullet. The second leading cause of leading (don't let that confuse you, okay?) is using an improper lube. There are other causes, but if you take care with proper sizing and lube, you won't have any problems.
You don't have to worry about using wheel weights. Unless they're the stick-on type and not the crimp-on. Or they're Zinc. Which really, really sucks.
It really depends on the type of shooting you are going to do if wheel weights are going to work for you.
What Quarter said hits the nail on the head. For most shooting, even up into the 1800fps range, they will do just fine. If you want to go any faster or use them in a semi auto handgun you may want to make them just a little harder.
Adding tin will do it but that is expensive. Try to find some Linotype. Go to a scrap yard and see if they have any. If so, adding 10% is all you will ever need.
Remember, Elmer Keith used a 16 to 1, pure lead to tin, ratio for most of his loads. A 10 to 1 ratio for some of his 45 AutoRim loads was the hardest material he used.
i have been an ihmsa silhouette shooter for 28 years, and have used wheel weights for most of
my cast bullet loads. i alway drop them in water, right from the mold, and it hardens them to
the extent that leading is reduced considerably. besides, they can be found at almost any tire
shop, and most of them are willing to selll them to you at very good prices.