It is important to use only fully synthetic (Group IV and V, rather than Group III) 5W-30 or 5W-40 oils in any Mini Cooper with an API SH specification or higher. Look for the specific requirements of your car using the owner's manual and then check the approved oils with BMW, which has its own system in place for oil regulations and requirements. Also, follow ACEA (Automobile Manufacturers Association) guidelines.
Castrol Syntec 5W-40
Syntec, made by Castrol, meets several of the criteria for ACEA standards and BMW LL ("LongLife") standards, including ACEA A3/B3 and BMW LL98. BMW's LL system relates to the longevity and functionality of the oil. In practical terms it means that you should drain and replace your car oil every 15,500 miles for LL98 vehicles and every 18,300 miles for LL01 vehicles, as explained by ACEA.
Valvoline SynPower 5W-40
Valvoline's SynPower meets BMW's main requirements, including the critical LL01 standard and A3/B3 requirements. Make sure you perform an oil change every 18,300 miles. This oil does not comply with LL01 in the BMW range, so avoid using it in models of the Mini Cooper that specify LL01-compliance.
Valvoline Max Life 5W-30
Max Life meets both ACEA A3/B3 regulations and BMW LL01, making it one of the most popular Mini Cooper oils. For an oil to meet the ACEA A3/B3 requirements, it must be stable, have a high viscosity rating at high temperatures, a low evaporative loss ratio, low levels of harmful, environmentally unfriendly sulfated ash and good oil/elastomer compatibility.
Mini Branded Motor Oil
There is a Mini-specific motor oil by Castrol called Mini Full Synthetic Motor Oil. It is a 5W-30 oil that comes in quart-sized containers. Castrol makes this oil from both foreign and domestic components in the US especially for BMW, and it meets all the ACEA A3/B3 and BMW LL system ratings. High performance 5W-30 oil with part number 07 51 0 143 829 is what professional Mini Cooper mechanics use in scheduled oil checks, according to Minie.
Mobil 1 SAE 0W-40
Mobil produces a 0W-40 oil suitable for use in Mini Coopers. The "W" in oil terminology stands for "winter" and refers to the oil's cold weather weight. It is dependent on the oil's viscosity at zero degrees Fahrenheit, according to Chemical and Engineering News. The lower the W value, the more suited the oil will be to colder climates. Factor location and climate into your decision when buying your oil, if your manuel suggests you have a choice to begin with.