Hello, what would the best viscosity(5w-30w,10w-30w,10w-40w,20w-50w,etc.) be for the 1975 2F motor? I plan on using fully synthetic. I live in Northern California(typical summer heat=105 degrees, winters=40 degrees, averages).Engine has either 81,000 or 181,000 miles, i tend to believe 81,000 because of overall condition of vehicle. I don't have a tachometer but i rarely rev this thing to high as i'm never in that big of a hurry to get anywhere in this thing. Off road is done at a snails pace.Thanks