Important facts to know...

Fat over lean...

One of the worst mistakes that artists make is to set up a cup of oily medium on their palette or stand and dip into it indiscriminately with their brush. If one medium uses 2 parts oil and another uses 3 parts oil, but you apply twice as much of the medium with 2 parts oil, then you have just added 4 parts of oil to your paint, which will be fatter than the layer with 3 parts that follows it. You will not, then, be painting fat-over-lean. You must apply the same amount of medium to your paints for each color and for each layer.