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It's just a guess on my part as I don't use the system but the salt and other additives are made after cooking. It should be just the oil and accumulated bits and pieces in the bulk oil.
Gunfrk is correct, MOST of the time salt is added to foods after being removed from the fryer. And going to higher end resturaunts to collect oil helps this also, since they don't just blanket salt their stuff, and the fryer oil is changed far more frequently. I am sure their is a trace amount of salt, but not enough that it should hurt anything. I suppose only time will tell.
Drawbacks to free fuel.... I may have to think on this one a while. The only drawbacks to a correctly installed system would be Initial expense, and switching back and forth between diesel and SVO. Of course, as long as it is above about 40, you will be able to start from SVO, just need to make sure to let the motor warm up. Small price to pay for essentially free fuel, and a cool smell from the tailpipe