Polyurethane foam comes in many different grades of density and quality. It is expressed in a 4 digit numbering system. The first two numbers are the quality of the foam, meaning the amount of urethane in the foam, and the last two numbers are the firmness, or density of the foam expressed in ILD which is indent load deflection. The higher the first two numbers the better the foam, and the higher the last two numbers the firmer the foam.
There is no reason to use softer foam for the top layer, because once you sit on the foam it will compress under your body weight making the softer foam much less effective in supporting you and much less efficient in holding it's shape. There is no original equipment auto seat foam that is more than one density, which is usually very firm. The support you want will be more dependent on what is under the foam to support it.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 01-10-2013 at 07:59 AM.