I had 1.8 gallon tanks and swapped them out for 3 gallon tanks- the bigger the better. They are Smartcraft tanks so they tell me the level and when they are low. When they show 1/2 full I fill them up and carry a 2.5 gallon jug onboard just in case. It's been a while since I ran the numbers but I think I burn 1 gallon of oil per motor for every 100 gallons of fuel. Boat holds 212 gallons so that is a 2.5 gallon jug for every fill up. I fuel up when I get under 100 gallons but never seem to fuel and oil at the same intervals. A cruise 10 miles away has turned in to a long weekend 100+ miles each way more than once so I like to be ready. That along with my boat runs better in the rough with the extra weight.
FYI- The motors have an oil reservoir on them and then the remote tanks. If you run low and all you have left is the tank on the motors, when you fill the remote tank you have to take off the cowling, crack the top of the motor mounted reservoir to let the air out and the remote tank will pump it full and then seal the cap again. Don't ask... At least the mechanic I called out at 2PM on a holiday weekend only charged me the minimum $350 for the service call to show me that...
See that triangle opening on the back right behind the cup holders? It has a tray in there and then the oil tanks under it. 3rd pic is the Smartcraft sensor thing. You can adjust it for any size tanks but I think 1.8 and 3 gallon sizes are a kit from Mercury but not sure.