You wouldn’t be able to feel, hear or see a difference between 5w30 and 5w20 in most cases.I dont like using the thin stuff but am forced to because of mpg concerns and trying to squeeze out the last bit. The 3.7 Liter Duratec calls for 5W20, but I have used 5W30 and 5W40 in the past. So I put SuperTech 5W20 Advanced in it and I notice immediately the engine is a bit peppier and faster. Seems to run better with the 5W20. My observations in the past was the thicker stuff quieted the engine down overall, but there was a bit of performance lag.
It seems like all my decisions nowadays are trying to increase the mpg with gas prices north of $3...lets hope the 5W20 can squeeze a little bit more mpg out of this engine.