August,
In a wall stud, either bar soap or wax works well. In a live tree, I would probably go with natural bees wax.
I have tried to find my old literature on lag bolts, but I was not able to find it today. Using a 1" lag bolt for ease of math, let's do this......
For every inch of diameter, there are 3.14" of circumference. This means that there is about 3" of inclined plane (Thread) being forced into the wood for every turn on the bolt. If there are two threads per inch of length, that comes out to about 6" of thread being forced into the wood for every inch of insertion. If the lag bolt is threaded 6" of its length, the length of inclined plane being inserted into the wood will be 3 times 2 times 6....or 36" of thread being forced into the wood. The friction on 36" of steel rubbing on the wood fibers can be very difficult to overcome, but it can also be very destructive to the wood.
If not done properly....not using lubricants and going too fast......the wood itself can be torn, ripped, shredded, burned, or otherwise compromised. There is a whole lot of heat being generated as the lag is being turned in. This heat can be excessive. As the heat is being generated, the length and diameter of the lag screw increases. As the bolt cools, it will shrink in length, tearing the wood fibers even further.
When installing multiple lags, alternate between them, making only a turn or two on each one before moving to another. This gives each lag bolt time to cool between operations. If wax is used as a lubricant, this will also keep the wax from thinning too much.
Hope this helps.
Joel