Oil for DB9 use a good quality synthetic
You didn't indicate the year of the DB9 but I believe that the owners manual (which should have come with the car) and the service manual stipulate a Fully Synthetic 10W-60 oil meeting API specification SL/CF or ACEA specification A3/B3/B4. I have found that 10W-60 is hard to find and while there are BMW versions available I don't think you need to pay a premium for them as the most recent street brand names like Castrol and Mobile have even better additive packages.
Several manufacturers make fully synthetic oils and with later (better) specifications API SM or SN for example. Since the 10W-60 is very hard to find the manual also stipulates 5W-50 with the same API or ACEA specification. Castrol, Mobile and others make 5W-50 with API SL/SM/SN (the SM or SN are the latest specs and have better additive packages) and meeting ACEA A3/B3/B4 or better. I don't know your location but if you are in Europe or Asia the oils are probably labeled with the ACEA specs. I wouldn't buy oils that are not clearly labeled with meeting or exceeding either API or ACEA specs. Again, you should check your owner's manual just to make sure. Also, do not use oils that are not fully synthetic as they are not compatible with your engine and seals.
Unless you are going to aggressively race this car you are not likely to need 10W-60. The 5W-50 has a viscosity curve that would handle most driving conditions (even hard driving)and temperature ranges. Hope this helps.