What is the difference between Decking Oil,Decking Preservative, Decking Paint and Decking Stain?

Oil penetrates the wood, and helps repel moisture. Preservative can be several things (read the container), but will help preserve the wood by either sealing or penetrating it. Paint coats the wood, and will help preserve it by keeping moisture out. Stain colors the wood, and may or may not help preserve it (again, read the container). Ask the staff where you purchase these for specific advantages of each. With any of them, be sure the wood is clean and completely dry before application.

For your weather, I'd go with a sealer, applied to the entire post, or treated lumber.

Stain is just for coloring your wood. It don't do much to protect the wood. The paint will also color the wood, but it will block some of the rain from getting through.

It sounds like decking oil and decking preservative is something to soak into the wood and protect it from the elements. I don't know if there is a different name for the product in England, but you need a water repellent. I use a product made my Olympic that I put into a pump sprayer. You need to give it a thorough spraying each year when the weather is dray and no rain is expected.

these products are used in order

1. Stain

2. Preservative

3. Wax or Oil

If paint is used, I believe the other products are not.