I changed the oil today in my '99 Ranger 3.0 today and goofed my weights, an amateur mistake. My Mustangs are 5W-20 and the truck is 5W-30. Will the thinner oil at temp be an issue? I ran the motor for a minute and it sounded fine. For a Ford engine with 270k, really the only noise you hear with it running is the fan. I did use full synthetic high mileage, and have for the last 4 changes, with the right oil previously. I've had no issues related to that.
I don't think your '99 Ranger will mind the small difference, especially if you say it runs quietly, and it's synthetic. Maybe just run a shorter interval.
I actually asked a coworker who's in his 50s and is a 3rd generation mechanic. He's never done that and run lighter than specified oil before. He said the winter rating is most important. Some stuff online says 5W-30 and -20 are interchangeable based on use and environmental temps and others say not. Haha. I'm curious if Scotty will weigh in on this one. I know he doesn't like full synthetic in high mileage motors. I've used Pennzoil Platinum High Mileage for two years with no noticeable changes