I believe the current recommendation is DFI oil. There is a sticker under the cowl that says what oil to use(or at least mine had one). Mine said premium plus, but it was a 98 model. I think since then Mercury changed and is now recommending the DFI oil.
2003 Ranger 521VX 35th Anniversary 250 Yamaha
1979 Ranger 168V 140 Evinrude - Papa Smurf