On archipelago there probably isn't a large enough area of desert/tundra/snow for oil to appear on those terrain types, so it appears on nearby coastlines instead.
If we acquire 2/3 of the oil in each region we control, and we conquer 4 people by the modern era, we can field 2 * 4 * 2 = 16 oil units. I usually have about 50/50 battleships and tanks. Aluminum supply is about 24, or two dozen aircraft.
When we're playing peacefully and only control one region, we can field 2-3 oil units and 3-4 aircraft. These resources aren't important in a peaceful game, however, since we have less use for offensive units (tanks and battleships). Rocket artillery, aircraft, and subs are better for defense.