Around Y2000, I was involved with biodiesel and waste vegetable oil. The precombustion chamber VWs loved the stuff. Biodiesel was better but WVO was a good second best. If there is a catalyst, the exhaust gas has no smell what-so-ever. Fuel consumption is about the same as dino diesel.VW TDI and PD were great. Others(?) who knows as most manufacturers would not warrant biodiesel. A friend needed the head gasket doing on a VW 2.4 showing nearly 200,000 miles. The engineer could not believe how clean the engine was and after doing the repairs set up his own biodiesel plant.
WVO must have hot fuel oil and it's own (heated) fuel filter or the injection system will not cope. You need to start on normal diesel and switch to WVO when engine is fully warmed. You need an additional accurate engine temp gauge to know when to switch. You also have to switch back a few miles before arriving so the WVO gets fully purged. Its a big hassle only worth doing if you do long journeys.
The recipe for biodeisel is here
https://www.answers-to-your-biodiesel-questions.com/Biodiesel-recipe.html
Assuming the engine and fuel systems are compatible there is no problem.
BUT -
Biodiesel strips waxy deposits from inside the fuel tank leading to clogged pipes and lift pump. You have to properly clean the fuel tank with caustic soda and hot water = tank off the car to do it.
If the car is not compatible you will quickly have leaking fuel seals and O rings. All rubbers go hard very quickly in methyl ester. Even silicone rubber can't cope. The ultra fine injectors of common rail systems might be ok as methyl ester is a good lubricant. Tell us how it goes.
2 litre pop bottles are (clearly) not convenient. You can make a successful batch processing system. But its a lot of work and you have to dispose the crud from the waste oil and the soap byproduct from the biodiesel.
Bulk methanol is hard to get today because its used for methamphetamine manufacture. Its also a nasty neuro toxin. Waste vege oil still has to be properly cleaned. The best way is boiled over water - you can imagine how that smells. Methanol and caustic soda mix is a paint stripper that will painlessly burn holes in your skin.
If you want to save money on fuel, a liquid phase LPG system is by far the best option. Cheap and easy to install though you might have to remove the intake manifold to fit the gas injectors. Gas phase systems are a bit cheaper but more hassle to fit and less economical.