you lot have probably heard me say it before but....
PM Lubricants- I'll back them all the way.
Why?
They test your oil for free every 5 or 10 000ks. If all is running well, and you are not doing short trips or if there is nothing else contaminating the oil, you will get up to 40 000k's on the one sump, after the first change (the first change there is always residual oil from the last lot)
Why I like them so much is because if you tell them you want to change it after 5 or 10 000k's, they actually explain what the oil does and, provided it passes testing, how long it is expected to last.
Quite frankly, an oil change is cheap and easy, at around $55 for 5 litres and a filter. The main reason for undergoing the full testing proceedure is to understand exactly what is taking place inside the motor- are bearings wearing, are you starting to run rich, is the air filter blocked? All of these factors can be found in the testing process.
My favourite test- run the oil you have now. Take it to a lab for an independant analasys (around $33). Then, change to PM and run it for longer. Take a sample to the same lab and get the same test done. They guarantee that even after doubling the distance travelled since an oil change, their oil will show markedly less wear particles.
And they have found most oil additives only last around 3000 k's before it has lost its effectiveness. There was one they said lasted longer but I cant remember its name. It wasnt common though