Unfortunately Volvo Penta no longer offers the QL bow thruster oil (41100129) and they recommend using Castrol Alpha SP 150 or equivalent gear oil. We don’t sell Castrol but your local Napa probably does.
Attached is the specifications for the oil. You may be able to bring that to Napa or another auto parts store and they might be able to find the right oil for you.
Attached files: Product Bulletin QL Oil.pdf