Can 8018 and 9018 welding rod be used in lieu of 7018?

I have a WHOLE BUNCH of this in VERY old unopened cans. Do you think its still any good? I can dry it in the oven

That was my experience as well.

The question is will welds of regular mild steel be "sound".

It depends on what you mean by "sound", and what the geometry of your weld is. Your 8018 and 9018 welds will definitely fail at lower deformation than a 7018 weld, and cracking will be more likely, but not necessarily a problem. In all cases I have seen where 9018 was required, preheat to 400-500F was also required, but these were multi-pass welds on ~2% chrome-molly steel. Why not try a few welds and give them the hammer test?

If there is a 3-letter code after the xx18 designation your rods are probably special purpose, such as for chrome-molly, and a preheat requirement (or a brittle weld) is more likely.

While the rods might work for your purposes, for any critical weld on structural steel, where failure would cost a lot or possibly hurt someone, I would use 7018 even if I had to go buy it.

OK, thanks a lot...

Looks like I will stick to my original use...

I take a bolt biter and cut both ends at an extreme angle to make a point. Then use it as a tree spreader to bend apple tree limbs down.