As stated above, Oil works best in terms of 81 when using the Recycled Loop.
Game doesn't work in ppm, game works on cycle times with respect to clock speed.
Clock speed is limited to xx.xxxx format, so any time you would have a repeating decimal or one that is simply longer than 4 places, you're not getting the amount you think you are because the game truncates without rounding.
The rule was found when I was looking for terms to put final products in where all machines prior automatically fall within that 4 decimal limit, so you would have to worry about un-clean numbers.
Solved first was Oil. Where we learned that doing things in terms of 27 Oil input was the cleanest. And that equates to things in terms of 81 output for Plastic/Rubber. Finding the literal lowest multiple was easy after that, and it is 10.125 (but again, 81 is simple. Just knowing the actual lowest helps you solve things like "if I needed 100, it's cleaner to make 101.25")
Same logic applied to solving other items in the game. Initially I went looking for each item's individual number, but discovered that all but 3 recipes fall under 45 (guessing some dev intentionally did this).
Hence, 45-81 rule.
Exceptions: Caterium Circuit Board, Fine Concrete, Pure Iron.