Most 23 & 28 Tender's are powered by an Evinrude E-tec. Either Generation 1 (G1) or Generation 2 (G2). They are both 2-stroke engines and need engine oil refill. 

Evinrude E-tec Generation 1 ( - 2014)
Evinrude supplies two oils for their outboards. XD-50 and XD-100. When the engine is brand new the technician can choose which quality the engine shall use. 

All Evinrude engines mounted by Goldfish Boat is programmed to run on XD-50. 

Evinrude E-tec Generation 2 (2015 - )
Generation 2 can be run on both qualities. XD-100 is a higher quality oil that BRP recommend to use in cold climates. At Goldfish, we deliver all boats with XD-50 and are happy with that. 

This is what Evinrude Owners Manual says about 2-stroke oil for Generation 2:  

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