Jim,

Sorry you are backing out on this.

As I see it, several of us are speaking of the same thing in slightly different ways.

If I understand Downunder's point, the flammables in the fuel provide the power. To get the same power, you have to burn the same quantity of methanol (and nitro, if it's part of the blend.) The oil content doesn't affect that, but in a low-oil% fuel, more of the methanol (and nitro) are in the mix drawn in by the engine in each revolution - presuming that all the fuel ingredients are vaporized or atomized to the same degree as they go into the engine. With a higher oil percentage fuel, you need more total fuel to enter to keep the same quantity of the flammables available for burning. Richer needle, in other words, and often more fuel consumed per minute.

And oil absorbs heat from the engine metal and from combustion. Burning is supposed to produce heat, and that heat is our power. By reducing the heat released, in proportion to how much the oil absorbs, and by taking up space that could otherwise have been filled with the combustibles, the oil content damps down the combustion and thus the power production. We live with that, since without oil, our engines would quickly become expensive, funny-looking, welded-solid paperweights...

These ARE definite effects on how a fuel runs. Adding more of any one of the main ingredients, without changing the others in the same proportion, also has effects...

To compare: If a fuel is 20% Nitro and 25% oil, the rest is the methanol - 55%. If it is 20% nitro and 15% oil, methanol makes 65% of the fuel. It is an interesting algebra exercise to find how much oil to add to bring its % up from 15% to 25%, because the oil added increases the total fuel volume.

Example: If you start by adding 10% of the original volume as oil, think of it this way. You started with 100 "%-units" of blend. That means 20 units of nitro, 15 units of oil and 65 units of methanol. Add 10 units of oil. Total volume of the blend is now 110 units. Nitro units don't change. Methanol units don't change. Their percentages do, however.

The 20 units of nitro in 110 total units of volume isn't 20%; it's 18.2%.
The 65 units of methanol in 110 total units of volume isn't 65%; it's 59.1%.
Even the oil % changes: 15 units + 10 units is 25 units in 110 total units of volume, which is not 25%, but 22.7%

Add up the new percentages: they equal 100% of the new total volume. And, you have that extra 10 units more than the original total fuel volume to deal with...

You can doodle out a few more guesstimates of how much oil to add to make 25% of the new total volume in a few more runs with the same procedure, and also figure the effective nitro and methanol % that result...

And, Bob, Hi! There may be a fallacy about the synthetics leaving the engine unprotected/unlubricated if they burn away during combustion... Sure, some part of the synthetic % does burn, but only WHILE it is exposed to the burning methanol and nitro. The power stroke of the piston, in other words. ...When the piston is moving down, uncovering the sleeve where nothing is sliding in it. The next bypass charge sprays the interior of the sleeve with a fresh dose of cool oil for the piston to ride up on during the compression stroke. Part of the reason synthetic blended fuels are popular, may be that many sport RC fliers prefer that their models do not become so oil-coated as we CL fliers are very used to putting up with.

The synthetics are excellent oils, but not for all uses. Similarly, all-castor as the oil% is not ideal for certain aluminum piston ABC/AAC/or ABN engines. Castor's tendency to form a varnish coating on cast iron pistons helps those stay lubricated and sealed; tapered-fit brass or aluminum sleeves have no room for such a coating...

Finally, if all else fails, re-read the engine manufacturer's instructions. Mfr's certainly won't tell you to do something that will make their engines look bad...