I have an old spout assembly for cans in my box, still in the plastic wrapper. I don't use it. They're messy and they can introduce metal from the can into the engine or oil system. I use a standard "church key" triangle can opener which pierces a triangle shape in the can. I punch one slowly in the top of the can, then rotate it 180 and punch another for a vent. If any doubt arises over the material being poured, it can be poured through a filter.
These openers, usually called "church keys," can be had anywhere that sells kitchen supplies. They're usually a dollar or so. They're flat, fit in a flight bag or just about anywhere, and clean up more easily than a spout assembly does. No mess. If I'm traveling with one, I like to keep it in a plastic sandwich bag to prevent contamination of the tool, and to keep any oil residue of my things.