I'm assuming the RTR spoiler is unpainted, matte black plastic.
If so, yes a product like 303 would be a good thing to use on this. You will need to apply it regularly as 303 is water based and will degrade with rain and washing.
For something a bit longer lasting, the ceramic spray sealants will work well here too. I used CarPro Reload on plastics like this, with a longevity in the months rather weeks for 303.
Your next option would be a ceramic coating for trim such as CarPro DLUX, Gyeon Trim or GTECKNIQ C4. These will give 12 - 24 months protection.