As bizarre as it may sound, the theory behind this was that there is a certain type of (cooking) oil which is fairly difficult to get rid of if you get your hands greasy. You would need very hold water to sort of melt it away from your hands and then use soap. If this oil is heated and applied to your body before sliding into a wetsuit then it creates a sticky layer which sticks to the body more than trapped water. If wetsuits work by trapping a thin layer of water between your skin and the neoprene then trapping a layer of oil is supposed to be easier for them.
I do not think it would work under a drysuit but it should work under gloves and hood while diving dry. I have never felt cold in the head area with hoods but I have found that under forty degrees I have to constantly move my fingers or my hand could go limp with cold.
Do a little experiment. Take a bucket of ice water. Heat some cooking oil (any oil but the stickier the oil, the better. I used olive oil because that is what I cook in). Apply it on one hand and wear a 3mm neoprene wet-glove or semi dry glove. The other hand should be placed in the glove without the oil. Now put both hands in the ice bucket and see which one gets colder after 5 minutes.
