If you're talking about the mid to late 80s engines, the most 'famous' of which is probably the BMW turbo four, there was zero diesel in their fuel blend. According to numerous interviews with multiple team engineers from that era, each team was developing and running their own blends but all the blends were similar in that they were 100% aromatic hydrocarbons.

The BMW blend was allegedly a 84/16 mix of toluene and benzene. Other teams used a blend with a similar toluene component but some amount of xylene as well.

The only reason I could think of they would add diesel to the mix would be to lower the octane to get below the limit of 102.

A BTX mixture. What were they thinking of? Horrible liquids.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

They were thinking of winning races, cancer in pit crews be damned.

FIA didn't implement a rule which required fuels to be made from the same compounds as commercially available fuel until 1995.

This brings up an interesting idea: Optimizing a mixture of two different types of fuel for different engine speed/load combinations to optimize combustion flamefront speed and/or reduce knocking.

Well, methanol water injection has been around since the 1940s. It was pretty mainstream in large reciprocating aero engines from then until their demise with the advent of the passenger turbojet. Since then its closest approach to mainstream was the Olds Turbo Rockette engine of 1962-63, but typical owners neglected to replenish the fluid, so Olds dropped the idea and swapped in a bigger normally aspirated engine.
Now commonly used in classes of racing where it is not prohibited, google "Aquamist".

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

I don't know, but it seems quite likely that this strategy might have been used with the Lucas mechanical Petrol Injection system widely used on racing engines in the 1970's & 80's.

The Lucas shuttle metering concept involved a steel rotor running at half engine speed in a steel sleeve and lubricated by the fuel. In the days before coatings such as DLC, this arrangement worked reasonably well but was marginal with the minimal lubricity of typical gasoline; with the even worse lubricity of Toluene it would be liable to scuff and seize. Adding diesel fuel could be expected to restore the essential (for this mechanism) lubricity to an acceptable level. The dosing rate was probably a guess based on empirical experience; today we would use the HFRR (High Frequency Reciprocating Rig) test to more accurately characterize the fuel lubricity.

(((((" It was pretty mainstream in large reciprocating aero engines from then until their demise")))))
Those engines are still in use today all over the world, no demise there. Everts air in Alaska uses those demised engines daily hauling cargo for profit, those engines have much better fuel economy and less expensive overhauls than a turbine AC engine.

Point taken, yes the large radial engines are still around, but in rather niche applications. No new aircraft design has used them since...???
Are they still using water/methanol injection for take-off power?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

I would think ADI would be used more now since the lower octane fuel they have to use now. Actually there was a new aircraft that used them some years back, for forest fire work.