This is a video on the fuel delivery of these Chinese Diesel Air Heaters.
Most of these heaters come with a 2mm bore, hard nylon, fuel hose plus short lengths of normal rubber fuel hose that are used as joiners. This is the genuine Eberspacher method of delivering fuel to these heaters, and has remained unchanged for some 80 years.
To lessen the chance of fuel leaks with all these joiners for the 2mm nylon fuel hose some people use straight lengths of 4mm bore fuel line, and now some of the suppliers of these Chinese Diesel Air Heaters, (Eberspacher copies), are now starting to supply their heaters with 4mm bore fuel hose rather than the standard hard nylon 2mm bore fuel hose.
Some of this larger 4mm fuel hose supplied with these heaters is a very soft vinyl type, and is just not suitable for these heaters, while some is a much better quality, harder type of fuel hose. If you are going to use the larger hose you should use the harder better quality hose.
Using the larger 4mm bore fuel hose is generally ok provided it is of the harder variety, the lengths are kept as short as possible, the fuel pump does not have a high suck fuel lift, and there are no big loops in the fuel hose delivery side where large bubbles can form.
With a good heater instillation, with no soot producing restrictions in the combustion air intake line or the exhaust system, these heaters will run very clean so there should be no real issues with using the larger bore hose, provided it is the better quality stiffer fuel hose, and not the very soft vinyl type of fuel hose.
The issues arise when you start to get carbon build up in the heater. The "soft" vinyl type fuel hose absorbs the pulses of the fuel pump and instead of the diesel squirting into the burn chamber and past any soot build up, the fuel pump pulses are absorbed by the soft vinyl fuel hose and the diesel "dribbles" into the burner chamber rather than squirting into the burner past the soot build up. If this happens your heater will not start or run well without a maintenance clean of the atomizing screen.
A real problem arises if you get large amounts of soot in the main burner atomizing screen which is near impossible to get at and clean by the average heater owner.
If you run oils in these heaters you will get rapid soot build up in the burner chamber.
It may be interesting to try running engine oil, vegetable oil, snake oil and gator oil, in these heaters, but all oils leave large quantities of soot when burned and you are setting yourself for some serious maintenance issues down the track.
You can not join into the vehicle fuel line if there is a fuel pump in or near the vehicle fuel tank. When your vehicle is switched off the little dosing pump can not suck the fuel past the vehicle's fuel pump, and when the vehicle is switched on the fuel pressure in the vehicle fuel line is too great for the little dosing pump.
Disclaimer. - As normal I only give suggestions, you must do your own research to find what is most suitable for you.