If synthetic cleans away deposits you’ll see consumption go up. We had a fleet of diesels do that after we put an additive in our bulk tank. Took a bit of time and then bam it hit and was shocking.

On zinc, Valvoline says you don’t have to worry about it with modern oil because other things in the mix take care of the issue. But they still sell you VR1 if you want and that’s what I use.