Most people experimenting with this say that 1ml/liter is enough, and adding more will not give any appreciable benefit.
Also note that the Acetone addition is cautioned against not because it gives no use, but because of the effects of acetones on various materials.
Fuel passes through various components like filters and fuel lines. 1ml/liter is high dilution, but over a period of 20,000kms or so effects can show if your fuel system has any components which are sensitive to acetone.
For example O-Ring in Tata indica is made of a kind of rubber(Buna-N most probably).
Buna-N O-Rings
Why do they use Buna-N? Because it has high compatibility with Petroleum based products.
So using acetone in Tata indica may lead to premature failure(lets say at 40,000kms instead of 70-80K)
To find out which materials react with acetone here is the list.
Cole-Parmer: Chemical Resistance Database
I understand most of you do not have access to materials used list etc., so I suggest you buy O-ring and a couple of seals commonly found from open market.
Take Pure acetone, and dip the seals in for a few hours.
This is pure acetone, so to have similar effect seen in your car you would need to run it for thousands of kms
My take on all this
1. Acetone can increase mileage. Infact not just acetone but many organic aromatic compounds can increase mileage by either decreasing surface tension or boosting octane rating. I google all this, and to confirm this talked to a friend of mine who is doing Phd in organic chemistry. He says, yes, you can improve octane rating and decrease surface tension etc., and improve the combustability. Not just acetone, they use many exotic compounds, some of which are very expensive. He has not done extensive testing as his expertise is not really in Petroleum, but he knows enough about all the alcohols etc., and acetone and stuff(He is going to be a Phd in few months time)
2. The issue is effect of a compound in fuel.
Here let me explain the chemical thing.
When you mix two liquids which mix you either get a solution, or you can have a chemical reaction, and there are other possibilities too.
Now lets talk about solutions. If you take pure citric acid highly concentrated, it will burn skin and dissolve lot of stuff. If you take it highly diluted it will not have any adverse effects. However if you dip your hand in a dilute solution for extensive period of time you will get a reaction like drying of skin etc., Normally you do not sit with your hands dipped in lemonade, do you? Same is with Sulphuric acid. Concentrated sulphuric acid will burn a hole in your clothes. I have had a shirt ruined by just a few drops of acid. Luckily it was thick otherwise lot else would have been ruined.
But take sulphuric acid highly diluted, it will not have any adverse effect for short periods of time.
Same is with Acetone. You are diluting it to a level of 1/1000 (1ml /liter). So its a very very weak solution. But continuous exposure to this weak solution will have an effect sometime.
So you may have some rubber part failing at 20K or 30K instead of 40K kms. That rubber part may cost just 20rs, in that case the savings and benefits outweigh the cons, but if that damage leads to serious damage, it will be $$$$.
That said, I think the petrol car people are safer using all these contraptions than the diesel car guys. For example any diesel engine can use bio-diesel but it will lead to corrosion of rubber and plastic parts. Therefore the Mercedes Benz Biodiesel test cars use a different material for their plastics.
It is due to this reason that commercial additives in the market come in two varieties, "Street legal" and "Race only".
Street legal ones are which do not have any corrosive effect etc., while race only can shorten lives of some parts. When you are running a race you don't care whether your seals will last 40K or 20K. After a hard race many consumables and small things are anyways replaced, so the motive is winning.
Also note that the amount of damage depends upon the time of contact that the solution has with the material. My biggest worry is whether fuel tanks are painted from inside. Most paints will dissolve in acetone. If you dilute acetone, it will take a longer time, but damage will still be there.
I recommend all long time acetone users to inspect the Fuel filters. If your car has dual filters, like first stage in fuel tank, that will be the one which will catch most of the junk.