If you are looking for the longest possible service life from timber impregnation, creosote is the product you need to use. It has a very high tolerance to fungi, insects, and marine borers and it also naturally repels water.
Creosote is not allowed to be used residentially and can only be used commercially. This means its common uses are things like piling, telephone & utility poles, and railway sleepers.
Creosote impregnation is performed in a specific high pressure plant. Depending on wood species, creosote can be applied using various cycles, such as Bethel (full cell), Lowry, Rueping, Double Rueping, Triple Rueping (German standard Din 68811).This sometimes also includes a Boulton phase. The liquid is heated between 90°C to 130°C for treatment.
Creosote is still (and will continue to be) widely used in many countries around the world, WEI Type B and Type C is authorised for use for the production/treatment of Utility Poles and Railway Sleepers throughout the EU (subject to individual nation authorisation) and WEI Type B and Type C are authorised for the production/treatment of Utility Poles, Railway Sleepers, Fencing and Tree Stakes in European Nations that are outside the EU.
Creosote is considered as a highly toxic chemical and may be banned at some point in the future. For this reason, some chemicals manufacturers are now offering creosote replacements, such as copper oil products.