I have read a few threads about making your own fuel for you Nitro RC car:
1. What's right amount of castor oil to use in the fuel, 12%?
I use 10-12% total oil content in car fuel; I use a 60%/40% blend of castor/synthetic respectively.
2. Can you use synthetic oils? I have seen people saying they have but which brand-which type
Yes you can - and probably should use at least some synthetic oil. It keeps the varnish buildup under control. I use Klotz Techniplate because it's cheap enough and available locally. I believe it's PN is KL-200.
3. Does it really require Nitro, people say they don't use nitro and just methane and it works fine for them or is that just for Planes-Heli's
Nitromethane is not required, but makes the engine much more responsive and easier to tune. Methane is a gas (unless compressed) and as such cannot be used in a liquid fuel. Pure Nitromethane can be obtained from Torco through eBay or Amazon for around $50/gal. This is how I buy it.
4. Where do you find the stuff to make the fuel? your local auto store or a chemical plant
I buy methanol from a local oil company that sells racing fuels. It costs around $3/gal depending on the time of year. I get Klotz synthetic oil from a local go-kart shop for about $12/quart. Castor I've gotten from SPLUBE.com for $25/gal, and more recently $17/gal at BulkApothecary.com. Nitromethane as mentioned above - $50/gal from Torco through eBay or Amazon. High cost for the bulk of the materials, but my per gallon cost for car fuel is $15-20/gal versus $38/gal from the Hobby shop.
5. Finally, is it worth it, I have a TRX 3.3 and CN-16 engines and I want to save some money of fuel
For me, it's beyond worth it to mix my own fuel. I save huge $$$. I spend less than $100 annually on fuel which is roughly 5-8 gallons of fuel. Figure the cost of 5 gallons of fuel at $38/gal plus tax for Hobby shop fuel! If your Hobby shop fuel is significantly cheaper than my shops, then it may not be worth it to blend your own.