Boat title says it is a 1978 Mark Twain. On the gearcase it just says OMC Sterndrive (no 400 or 800) with a 235hp on a sticker below that. According to the Clymer's Shop Manual that I have, the 235hp set up was for 1977 models. Tag on the gearcase says Model# 983489 - Serial# J351978. So does this make it a Electromatic gearcase? The change happened between 77 & 78 so I want to make sure. If so can I use Lucas 80-90W high performance gear oil?