Oil pressure. If you ever were to leak, or have a clogged sump, that would be a graet indicator. You already have water temp on the stock cluster. I have oil temp/press and boost. Need to add EGT and Air/fuel ratio. Oil temp is pretty useless, as I'm in LA. EGT and boost are definetely a good dcoice. IMO Oil pressure would be next, unless you can find a very good and ccurate air/fuel gauge if you plan on tuning yourself.