So I see a chart depicting the 318 series broken down thus:
318-1. . .Standard Engine. . . . . .used on D100 thru 300 series
318-2. . .Semi-Premium Engine. .used on D400 thru 600 series
318-3. . .Full Premium Engine. . .used on D400 thru 700 series
What's the difference between the 3 engines?