When I first moved to my oil heated house 6 years ago I had no clue. Called a few of the local providers they gave me information like how to work out big my tank was and minimum order (usually) of 500l. I got 500l delivered. At the time I think it was 37p a litre (exc VAT) and when the guy delivered he advised me to get another 500l as it was cheap.
I watched the price of heating oil fluctuate around 40-50p a litre putting 500L when I had to or if I felt it was cheap and had room tried to fill up.
Then it fell to 14p a litre around the time of the first lockdown. I managed to squeeze 500L in then!
I needed to put 500L this January. Grudgingly did at 59p a litre, glad I did now.
Currently using the https://www.oil-club.co.uk/ they seem to be a tad cheaper than Boiler Juice and if nothing else the weekly email about the price they are delivering at is useful.