I know from experience that heating oil is all over the shop at the moment but crude oil drum price looks to have settled back to end-21 levels, maybe the manufacturer's will eventually reduce their prices back to reflect this...
Fwiw - on Morris' site at the moment, C1 work out to about £10.77 a litre if bought in single 5l, or £8.42 if you buy the 25l biggun.
https://www.morrislubricantsonline.co.uk/multivis-adt-c1-5w-30.html
I might try a kit with the Miller oil, buying filter and oil (in bulk) separately will be about the same price in the end, but without the large initial outlay.