I know this has been done a bit on oil,
Currently using castrol 4T 10w40, I thought it was semi synthetic but it looks like its fully synthetic, I need to check the bottle of whats left from the last oil change.
I bought in halfords at the time (although on their website they show 4litre containers of castrol 4t, I never saw them in the store) , they have a 5 litre of part (semi?) synthetic 10w40 of their own branding, any worse?
Where else to buy for bike oil?
I got filter in cotters at the time, but of the two places I went, neither were pushed about getting other items, like o-rings, which’d have to be replaced at some point.

As for parts, where’s/what’s best? Id usually have ruled out main dealers for car consumables and parts, but recently came across cheaper oil from VW for a car and have come across cheaper parts from Ford and Fiat for parts than spurious in the past, so maybe main dealers??

So seals, filters, ebay? main dealers? depends?