Hey DD not sure what type of waterslide you have...
I'm presuming he's using "inkjet" waterslide paper and creating the logo with an inkjet printer.
Inkjet printer ink needs to be "fixed" (sprayed with a fixative) to make it water resistant, otherwise it will dissolve right off the film when palce in water.
Laser printed decals (on laser paper) do not require this step.

...do i spray an oil based lacquer?
As for "oil based lacquer", it not a really a thing. Lacquer is a solvent based material which is different than oil based (one could argue that mineral turps is also a solvent, but that's another discussion).
The typical recommendation for fixing inkjet decals is acrylic lacquer (eg: White Knight Clear Acrylic). Most art fixatives are acrylic sprays. There is the whole acrylic lacquer vs acrylic enamel discussion, but that still confuses me.

As for Tru-Oil over the acrylic lacquer, I have no direct experience but I'm sure somewhere on the web someone can confirm or deny the compatibility. Tru-Oil has a proportion of polyurethane in its mixture and the general (very general) rule of thumb is: Lacquer over Poly, OK - Poly over Lacquer, NOT OK.

Since the fixative layer will be very very thin on the film, it may not be an issue, but I would research it more to be safe.