We heard that this product only needs to be used twice a year, does anyone know if it says this on the bottle?

So?? Which came first?! My bottle says 6,000 miles… this bottle is about 10 months old. I’d really like to know if the manufacturer recommends 6k or 6 months! Thanks

Would this product be suitable for a 1.6 Diesel engine in our family car?

I use it it a 1.9 TDI and 2 7.3 diesels.

Has anyone used this with a TDI with DEF / Urea?

It is specifically designed for restoring the performance of diesel engines so yea it will be fine in your TDI just make sure you follow instructions don't mix it too strong or your car will run like a raped ape for a minute

I want to use this in a diesel parking heater to try and remove some of the carbon buildup inside the burn chamber. Do you think this would do that?

It is very unlikely that using Diesel Extreme will remove carbon that is already built up in your heater. It is best used as a preventative to combat the amount of soot/carbon build up. Another note is that unfortunately, Diesel Extreme is very odorous and using it in an open heating element may cause some unpleasant odors.