It didn`t run smooth at all and the engine was louder. I drained it almost immediately(I know,what an expensive experiment!). 15W50 Mobil 1 is incredibly smooth. It`s not uncommon for it to remain over 100F here in Texas. My owners manual also warns in bold print to not use 5W30. Glad I didn`t leave it in and I`m also glad it was on a cool day.