Mixing 100LL with gasoline

[FlyingRon]

Both the popular Autofuel STCs allow any mixture of 100LL and (allowable) AutoGas. I've never heard of it being a problem. As Dana and Ron said, the biggest issue is that 100LL will lead choke an engine that was expecting 80.

[cub builder]

I usually run a mix of 80 - 90% 93 octane alcohol free Mogas with 10 - 20% 100LL. I mix the fuels in a 110G tank/fueling rig I have in the bed of my truck. I've got roughly 1000 hours running this type of mix. I do it primarily to keep the lead content down on the O-200 in one of my planes since 100LL has roughly 4 - 5x the amount of lead contained in the 80 octane fuel the O-200 was designed to burn and causes premature valve guide wear, but also for the direct economic reasons of the fuel being less expensive. Both of my engines (O-200 & O-320) are 8.5:1 compression. I have been unable to detect any difference in performance or fuel burn between straight Mogas, 100LL, or a mix. However, since I started running a mix of fuels, I never have to dig lead out of the spark plugs during the annual, and have had no issues with sticky valves or excessive lead build up on the valve stems in the O-200.

[1600vw]

In my vw I use a mix of 100LL and non-ethonal auto fuel. I add a product called TCP to the 100LL to keep lead at bay. When checking for water I like to see my fuel with a blue tint. The reason I use 100LL. First these engines were designed at a time when lead was being used in the fuel. Second 100LL will not vapor lock like auto gas. Now does mixing these two fuels still protect from vapor lock, I would like to think so.