I reviewed the blueprint for clean drives on Inara. It stated that grade 5 clean drives has a -10% optimal mass and a +28% optimal multiplier. Does this mean that engineering your drives to grade 5 clean with slightly increase or slightly decrease you jump range? Thanks
Neither.
The optimal mass parameter on the thrusters defines how your thrusters will interact with your ship's total mass in regular space, so how fast your ship can fly/boost/turn.
The (completely independent) optimal mass parameter on the FSD defines how your jump range will depend on your ship's total mass.
And the (completely independent) optimal (hull) mass parameter on the shield generator is one of the attributes that defines the connection between hull mass and shield strength.
As mentioned, though, the mass of your thrusters will add to the totel mass of your ship and therefore indirectly influence the jump range - but neither dirty nor clean thrusters change the thruster's mass. Thermal Spread gives you a 5% increase in thruster mass (decreasing your jump range), stripped down gives you a 10% mass decrease (increasing your jump range).
And as
@CMDRQuainMarln said - there's only some extremely limited value to clean drives, for >90% of all builds, dirty is the way to go. IMO, the exception here are ships that need a low heat signature in real space - smugglers and pirates for me. It's fun to have a cold DBX as NPC pirate, and then get the NPC comms complaining about there being nobody out there...