dear bmwers,i have recently bought 2 shell helix ultra (5w-40) and went to change my Engine Oil at a workshop nearby but was told that they only change their in house brand of oils (15w-50).can you guys recommend me any good and reliable workshop that is able to assist if i bring in my own oils ?was also told that my e39 2.5cc need 2 bottle of engine oils by my previous workshop. but i remember reading somewhere that my engine only requires 6.5 litre..am i being ripped off ?thanks !! 