There are two primary bases used in e-juices, vegetable glycerin (VG) and propylene glycol (PG), and a decent article on the difference between the two can be found here -> http://vaperanks.com/propylene-glyco...he-difference/
The point of VG being thicker than PG and producing phlegm has definitely proven true in my case but I much prefer the VG flavor and throat hit to PG so I mix the two types.
As far as nicotine levels I was smoking 4 packs of cigarettes per day and I started vaping with 24mg/ml and I noted no nicotine withdrawal symptoms. I've since reduced the nicotine level to 6 mg/ml and I just purchased some e-juice with no nicotine.
I'm trying to identify the primary triggers that prompt me to vape such as coffee, after dinner activity, going to bed, getting out of bed, etc because my last step will be to replace vaping with other hand to mouth habits like eating peanuts, sipping water, toothpicks, suckers, etc.
I'm not going to this level because vaping is bad I just want to use more socially acceptable habits, reduce dependency on more costly products while increasing dependency on readily available things like toothpicks, etc.