Hi, We use an old dustbin with waste oil and a drop of diesel added. Left to soak for a few weeks. Most have lasted over 25yrs. Track bed is crushed lime stone (not ‘Crusher run’ too much dust and fines) so is relatively free draining. Even under trees where lots of debris falls the sleepers have lasted a very long time.
No need in our case to weigh them down in the oil – we just turn them end to end (they go in verically)
Our track is 9 1/4 and sleepres are 3×2 ish – usually second hand timber un-treated.
So long as you treat the cut ends – as tanalising only seems to penetrate a few mm’s.
Biggest benefit has been keeping the track and sleepers off the soil / weeds etc.