Testing on an early 2000s Subaru Forester with 250k km on the engine.

This engine clearly has piston slap.

One common suggestion online to address piston slap is to use Shell Rotella oil (preferably at a heavier weight, like 0w-40). Here we test that theory to see if it makes a difference (or not) in the cold-start piston slap sound.

Baseline piston tap recorded with Mobile 1 Synthetic 5w-30 in the engine at cold start.

Tested the next morning after having changed oil the previous day with Rotella T6 Synthetic 0w-40.

You be the judge if there's a difference in the sound pitch or volume of the piston slap.

2021 UPDATE: A little over 2 years and about 60k km later, this car still has the same piston slap, but is running just as well as it did when the video was made. Piston slap is just part of life on these older Subies. :)