There are a variety of treatments available for external wood and in this country we tend to look for translucent finishes that show off the grain of the wood and it is these that I shall focus on (although there are actually some excellent opaque finishes available in eco-format as well).
The decision on what to use is largely dependent on a number of factors:
Has the wood already been treated with a product (stain, paint, varnish)?
What type of wood is it?
Will it be in constant contact with the ground / water?
How exposed is it to the sun and weather?
Do you want it to weather or retain its original look?
Is it located close to a pond / water, or will it be used in housing pets / bees?
If wood has already been treated it is important to know what it was treated with (so remember to make notes on this type of thing when applying as most people will forget over time). If the wood has an oil based coating it might be that you have to re-use this product (unless the product is really well weathered and the preservative has been really leached out of the wood). So for existing coverings your choice is now really limited if you want to go down the eco-route.
Some woods require certain treatments, so a hard wood generally requires a thinner preservative than a soft wood as the grain is tighter. Also some woods have natural tannins or oils that might react with certain products. So if in doubt use a wood oil that matches the timber. Larch oil for larch for example.
For timber that is to be in constant contact with the ground it is important to treat the wood prior to it being placed in situ. I would also recommend leaving any preservative to soak into the end grain overnight to really give this most exposed of areas a chance to soak up protection as much as possible.
For wood that is really exposed to the elements there is a need to use treatments that will help it to stop expanding, contracting, splitting and degrading. You may wish to protect the structure with a ‘disposable shield’ like some old pallets or plywood in really exposed conditions. Using really high quality treatments in highly exposed areas is important as it will save you a lot of time in the long run from having to re-treat every year. This is also true for locations where access is an issue.
If you want your wood to retain its original colour then using a UV stable treatment is important. Clear treatments are not generally UV stable as they have no pigment to protect from the UV light. However there are some companies who make clear treatments with a UV protector in. Notably Osmo.
Many treatments are not friendly to wildlife. They are designed to protect against things like mildew, rot and insect attack, so if you want a treatment that does not contain biocides etc then you need to be careful in what you ask for. Natural products are not necessarily safe for wildlife as nature has its own toxins. So using treatments that are safe for your pets / bees / ponds etc is possible, but you will sacrifice a certain level of wood protection unless you treat the wood prior to installation and seal up water based biocides with an oil top coat.
Generally we look to Osmo for their wood treatments they have a huge range of products for all situations. Auro though do a very good water based wood stain range and the earthBorn Varnish is very safe to use although not as durable as the Osmo. For more info contact Osmo or the Eco Home Centre for advice.