I didn't know Bunnings sold car stuff, but here we are.
Valvoline Engine Armour Engine Oil 15W-40 5 Litre, currently $19.89. Appears on it's website so likely available nation wide. I saw it at Bunnings Maribyrnong (VIC) specifically, at the front of the store next to the Returns counter. As a companion, it is currently $48.99 at SuperCheap Auto.