Complex sentence: The final product is a Petrochemical Fuel that will burn in most older Diesel engines with no modification, however, this fuel may be "heavier" that is less viscous than normal Diesel Fuel and may not flow properly in colder climates.
Split into simpler sentences: The final product is a petrochemical fuel that will burn in most older diesel engines with no modification. However, this fuel may be "heavier" (that is, less viscous) than normal diesel fuel and may not flow properly in colder climates.