Bogmasters right. Don't use any salt or preservitive at all. If you put any kind of preservitive in with the flesh bait you will only end up with the same product you started with only in a "fixed" state, meaning it won't break down. Fresh bait is good in the early season, but you need something with a little bite once the cold weather moves in. To make a good fish oil you need a good oily type of fish such as carp or suckers. Cut the fish up into managable pieces removing the entrails and place in a large glass jar or jug with cheese cloth or some sort of material to keep the flies out, but that lets the gases escape. I have used 5 gallon buckets with lids with a hose attached to the top and put in water. The gases bubble up through the water, but flies can't get in to lay eggs in the bait and ruin it. Place in the sun in a safe place where pets can't reach it but that it gets a good amount of sunlight and let it set for a couple of months. It is best to do this as soon as spring has sprung or early summer. The longer the flesh has to simmer the more oil you will get. After a while you will see the oil on top of the flesh. I use a turkey baster to remove the oil. I put it into a clean quart jar. This is your fish oil. It takes a while to get enough oil to last through the season, which is why I gave up trying to make fish oil and just buy it. Plus it keeps the wife happy that I don't have anything laying around stinking up the house or yard. Fortunately for me the trap shed it on the down wind side of the house so she doesn't know what I have cooking down there.
Meat such as deer or cow can be made into very good bait as well, but it needs to be done in a totally different manner. I like to have a slight taint to my bait so I let it set outside for a couple of days in warm weather. I don't want something that will gag a maggot, just something that will grab a K-9's attension. Once its reached that level I stop the rotting process with a number of different products such as salt, sodium benzocoate or borax. Than I add whatever other ingrediants I want to doctor up the bait a little. Castor, fish oil even beaver saq oil make great additives to bait that really fires up a predator curiosity. There are a lot of good forums and books on the subject. Just look around.