Is it unwise to use 5w or 10w30 in a car that calls for 5w20 if it has high mileage like 225k? Honda element 2003 is starting to use a little oil. I have always used synthetic and changed at 5k. My other vehicles call for 5w30.
I currently own a 2007 vehicle that is sped'd 5W20 oil. Back in 1997 when this engine was first produced it was spec'd 5W30. The engine was made from 1997-2008 without EVER changing tolerances or clearances.
In 1999-2000 for CAFE reasons ONLY, the manufacturer started to spec 5W20 for said engine to save gas mileage. The previous years' same engines were also back spec'd.
So, no, you don't HAVE TO use 5W20 in an engine spec'd 5W20.
BTW, I'm currently running 5W30 in my engine and sometimes 10W30.