G'Day Carole, and welcome to CF!
As to your query -- I assume that you really mean silicone, not "silicon" which is a mineral element. One has to wonder which of the thousands of silicone based compounds your friend means, how he proposes to introduce it to your MSD system, what he thinks the effect upon the holding tank
might be or, if a straight through system, what the effects upon your local environment
Many people routinely flush vegetable oil
heads, both to lubricate the pumps and perhaps do something magic to prevent salt
buildup in the hoses. The practice has mixed reviews
amongst cruisers. Substituting silicone based oil (likely what your friend is proposing) would replicate the lubrication, perhaps improve upon it, but would have an unknown effect on conditions in the holding tank
. I rather doubt that it would somehow attach itself to the internal surfaces of the plumbing
and "promote a smooth surface throughout". Many silicone oils have a specific gravity greater than one and are not miscible with water
, so would tend to pool in the bottom of a tank and just lie there.
Frankly, I would view this practice with great doubt.