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Is it acceptable to use 100LL and 100 grades of AVgas mixed into the same tank, or should one type of fuel be drained prior to addition of a mismatched type?

This is assuming the aircraft is approved for the use of either of those grades.

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    $\begingroup$ I don’t believe 100 grade fuel is available anywhere. Only 100LL. $\endgroup$ Aug 10 '21 at 20:02
  • $\begingroup$ @MikeSowsun fair enough. I've never come across it either. I've had someone tell me that if you mix them, that the colors neutralize and the fuel becomes clear. I don't know if that's true or an urban legend. $\endgroup$ Aug 10 '21 at 21:04
  • $\begingroup$ We used to mix green 100/130 and blue 100LL back in the 1970s and the result was definitely not clear. Colour charts say mixing equal parts green and blue yields cyan. $\endgroup$ Aug 10 '21 at 22:42
  • $\begingroup$ @MikeSowsun Shell claims it’s still available at wholesale, but I’ve never seen it anywhere at retail. $\endgroup$
    – StephenS
    Aug 11 '21 at 19:20
  • $\begingroup$ @Mike Sowsun: That's only if the coloring agents remain unaffected by the mixing. If there's a chemical reaction between them, the result could be colorless. $\endgroup$
    – jamesqf
    Aug 12 '21 at 17:52
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You can mix them freely.

While the modern meaning and measurement is far more complex, octane rating originated as the percentage of actual octane (C8H18) in a sample of gasoline. This means if you mix equal parts 100 octane and 80 octane, the result is 90 octane.

The addition of lead makes gasoline behave like it has more octane than it actually does, which is how you can get octane ratings over 100. (Ethanol does this too.) However, that doesn’t change the simple math above. If you mix two gasolines that are both rated 100, even if they got there by the addition of different amounts of lead, the result is still 100.

Also, the amount of lead in the standards for both 100/130 and 100LL is a maximum, not a fixed value. Actual retail samples today have significantly lower lead content due to starting with higher octane unleaded, so far less lead is needed and in practice 100/130 is now likely to be 100LL just with a different color dye. It’s simply not worth doing separate production runs for each due to the enormous fixed costs to clean up the lead contamination of the refinery, tanks, trucks, etc. It’s the same reason you can’t find 80/87 or 91/96 avgas anymore: there’s no point when they can just sell 100LL to everyone at a lower total cost.

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  • $\begingroup$ Actually it is not quite a simple algebraic relationship. In the example given it might be 90.1, not 90 , but of no practical difference. $\endgroup$ Aug 12 '21 at 16:40
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    $\begingroup$ @blacksmith37 Do you know the correct formula? $\endgroup$
    – StephenS
    Aug 12 '21 at 19:46
  • $\begingroup$ Each component can have a slightly different response to lead. And each gasoline contains several compounds in different ratios , so pretty complicated. But auto fuel today ( in US) does not contain lead so there is no need . I guess there is not much incentive to do it for avgas. Each refinery blend would be tested for octane with lead. A refinery makes different blends ; high elevation and high temperature need lower octane ( with natural aspiration). $\endgroup$ Aug 12 '21 at 23:53
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The obvious answer would be: Check your manual. Technically, there shouldn't be any problems. However, if your engine is designed for 100LL and you use the mix over a long period of time, you might want to have your sparkplugs checked.

Apart from that, the environment is definitely not going to thank you for using leaded fuel...

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