Rust
It’s been simmering since Dad caught up with them-now it’s reached the full boil of the 'Sam's pushed Dad until he snaps' phase. Dad doesn’t engage-won’t let a fight delay this hunt. Instead he takes aim at the target that won’t shoot back.
“Dean, why don’t you touch up your car before you get rust. I wouldn’t have given you the damn thing if I thought you were going to ruin it.”
The barb lodges in Dean’s chest and Sam laughs as Dad walks away. Dean smothers it, smooth as his baby’s finish from a lifetime of practice.