iv had my current 250 for coming up to 15  months. i had it serviced last year and used 5w30 oil, but i was wondering if i could use 0w30 oil as its service will be due later this month or next month. obviously it will be low SAPs diesel oil because of the DPF. i have known different manufactures that use the same engines, such as Volvo and Ford,  use different grades of oils in the engines

"WHY"

What’s wrong with the stuff specified in the handbook?

Thinner oil, better fuel efficiency.

When the engine was designed 0w-30 c1 probably didn't exist.

I used to run a Ford Focus st mk2, which shares the same block as the rs. The st uses 5w-30 as standard but most tuned ones are run with rs-approved 0w-40. My understanding is that the cold temperature viscosity is lower but high (working) temperature viscosity is a bit higher. This provides less friction at startup but better protection when being worked hard.

Providing it is low saps, 0w-30 should be better for a car that does short journeys, however I am happy to be corrected by an expert.

https://www.opieoils.co.uk/p-205731-total-quartz-ineo-first-0w-30-advanced-synthetic-engine-oil.aspx

The above looks interesting and even had a jlr approval.