I found this quote from someone who wrote to Shell. "I put this question to Shell who stated quite unequivocally that at one time Red diesel was in fact identical to road diesel with a red marker added for tax identification purposes. Changes in Goverment requirements for more stringent standards for road diesel have meant that sulphur levels have been lowered, along with other modifications to road diesel. However and most importantly, Red diesel/heating oil/gas oil has remained exactly the same. Andrew Morris has kindly confirmed that this has certainly been the same for at least 10 years!" Basically, red diesel is the same as normal diesel of ten years ago. If a car could use it then, it can use it now, however, a newer engine would be spec'd to run with less sulphur. Stricter emmisions testing would also cause problems.