I’ve got a 2002 Acura RSX with 223,000kms on it. It takes 5W20 oil but my mechanic puts in 5w30 during oil changes. When I asked him why he told me since it’s an older car with lots of mileage it’s better to use thicker oil. Is this true or should I tell him to stick to the 5w20 from now on.
Stick to what the manufacturer recommends in your owner's manual, or any updated technical service bulletins that they put out for your engine. Wrong weight oil can mess with your variable valve timing system and cause unnecessary wear because the pump has to work harder to push heavier oil through the engine. 5W-30 isn't much heavier than 5W-20 so it won't hurt anything right away, but use the manufacturer specified oil and a high mileage formulation if you're concerned about mileage.
With my own recent experience, my 1999 Ford Ranger has a 3.0L V6 and I should have used 5W-30 in my most recent oil change per the owner's manual. I mixed up the 5W-20 spec in my 2017 Mustang with the truck's and put 5W-20 in, instead. I almost drained the oil the same day and replaced it with the owner's manual spec, but after posting the problem on here and digging through Ford TSBs, I found that they updated the owners manual for my engine after 2002 to 5W-20 and back spec-ed it to 1993 trucks so I've left it alone.
If it doesn't leak, doesn't show low oil pressure, or burn oil significantly, then there's no reason to use thicker oil. It's a bit of an old wives tale.
I'd just stick to what is recommended by the manufacturer.
