With a very minor background in chemistry and a heavier background in aquariums, I say the only thing that you should be using, if you're using soap, is glycerine, nothing else.
PEARS TRANSPARENT SOAP would be an excellent choice. It is just Glycerine and a little cedar and thyme for pleasant smell.
Dish soaps have numerous ingredients in them and also are extremely viscous which makes me worry that they might leave residue. Glycerine molecules stick to water very readily and the soap rinses super clean. It also doenst really absorb much moisture like normal milled soap does- not that this matters- you're washing keratin scales, but it can't hurt that its this gentle.
Shampoo is sodium laureth sulphate, in some concentration or another. I really don't think that's going to do much for you in the way of "washing" your snake. It very well might- I mean your hair and his scales are made from similar materials, but I would stick to the glycerine IMO- and I'd wash a snake in a designated rubbermaid/bowl. Not in my sink or tub. Safety first... Toothpaste residue, etc. You never know.
Oil=NOOOOO I tried this when I was 13, might have done it too many times in a row but I dried this pooor pythons skin RIGHT out. He actually shrivelled. His next shed SUCKED and he drank like a fish for a week.
Don't oil a snake unless you're killing mites or ticks or something...
Smell wise I suspect your smell is being caused more likely by a minor fungal or bacterial culture on the snake, rather than the snake itself being "dirty". What might work REALLY well is simply to wash the animal in very acidic water and suddenly make it obscenely alkaline- murdering any bacteria or fungus, and not even affecting the snake.
To do this, take some water, add about
1 ml of phosphoric acid to about 1 1/2 sinkfulls of water. This should put you about a Ph of 4 and change. Put the snake in that ofr about 20 seconds, then add a bunch of baking soda, about 10 heaped tablespoons. This will jack the Ph to over 9. Dont leave the snake in there for more than five minutes. If he acts uncomfortable ( I cant see why) remove him. This is guaranteed to kill any bacteria/fungus and solve your problem if they are the cause
(Get phosphoric acid as PH DOWN from a fish store, or as phosphoric acid from a head shop, hydroponics, or garden store)- $4-8 CDN for a huge bottle....