This scenario is very similar to changing the automatic fluid on early V70 & many other cars:~ (gradual change)
If you go for a larger supply (20L) of the appropriate oil from companies like:
https://www.smithandallan.com/
https://www.westwayoils.co.uk/
You can then take out some of the costs whilst choosing the correct specification for the car.
Trust it helps.
Bob.