I know we have discussed oil change intervals and filters but I could not find discussions on viscosity. My manual (which is usually my bible) states I should use Ford Motorcraft 5W -20 engine oil. My Ford dealer said "things have changed since my manual was printed in 2014". I have a 2014 Mustang Convertible (premium and pony packages) with the 3.7 engine. The dealer invoice stated Ford Motorcraft 0W-20 "full synthetic" was used. I asked for an extra quart and they sold me 0W-20 Mobil "full synthetic" and said that is what they are now using. So - did they use Motorcraft "full synthetic" or Mobil "full synthetic" and does it even matter which "brand" they used? My opinion is that Mobil "full synthetic" is lower quality that Mobil 1 which uses higher synthetic base components. There are 43,000 Km on the engine and I have driven 1000 Km since the oil change and there is no sign of oil usage. There are no audible sounds that concern me like clacking tappets etc. Any thoughts appreciated.