No, it makes a horrible choice for a wood preservative for a wood deck. The environment is screaming at your idea. Rain would make the oil run off into the soil and that oil would pollute the water table as it soaks into the ground.
Recycle your motor oil and use Thompsons Water Seal or a similar product all available at Home Depot or your local paint store, on your deck.
Used motor oil has been used for wood staining for decades. Farmers used it to stain sheds and barns, as well as the wooden beds of trailers and pickups. It looks pretty good, and lasts longer than many commercially-available stains; it's not uncommon to hear about trailers in which the wood is still sound after 70 or 80 years. Simply mix it 1-3 parts used oil with one part diesel and paint it on, then wipe off the excess. The diesel thins it for application, but then evaporates off (just like the thinners in any paint or stain.) Make sure you don't contaminate the groundwater while you're applying it, but once it's in place (assuming you don't use so much that it can't be absorbed into the wood) there should be negligible problems with runoff. However, you probably wouldn't want the EPA or any "granola"-type neighbors to hear about your project, just the same. While a used-oil stain is good for many outdoor projects, I don't know if I'd use it for a deck, simply because you're more likely to be in physical contact with a deck than, say, a fence or he walls of a shed. Contact staining of clothing (or dirty footprints in your house) could be a problem.
Dan is right in what he says about motor oil but Thompsons Water Seal is for brick walls, not wooden decks.
Get a proper deck preservative.
Even though you asked this in boating, I assume you mean a porch-patio deck and not a boat deck.
If your deck is already pressure treated, you are good against rot and bugs for ten to twenty years. If you want additional protection, buy Borate powder and mix it with water and slosh it around. Available at Jamestown Distributors.
If you just want a sealer to look good, much as I hate HD products, their Bear sealer part number 60 something…….is excellent. I just stripped, cleaned and stained thirty year old in the tropic PT wood deck and it looks fabulous.
I use it on the side of my barn- termites really hated it when leaded gasoline was in use. Not very good for decking though as it makes the wood soft and it rubs off with foot traffic.
Hell no, why would you even think about that? Buy a preservative that is made for wood decks.