mustangrich

At some point at the turn of the century Ford went from recommending 5W-30 to 5W-20 in a lot of it's engines. My family has had several 3.8L's and mine is the newest (a 2002 Mustang) and for a period of time I put 5W-30 in it out of habit. One day at around 50k I looked at the oil cap and it said 5W-20. So just said, "Huh...ok," and started putting 5W-20 in it. I have a rudimentary understanding of what the differences are in the two oils (operating temp viscosity) but my questions are: Why the change? and If you continue to run 5W-30, doe sit make any difference? I didn't notice any difference at all when I switched. And last, if I switch back and forth between the two (let's say same brand/type of oil, but switching between the two weights) will it make any difference?

Colorado_Mustang

There is a difference in the tests that manufacturers do for fuel economy and emissions. You're not likely to see any real world benefits, however.