Yes.  The best oil additive I have used is Dura Lube.  The moment you put just a cup (8 ounces) in your oil you will notice engine noise decreases, fuel mileage and power increases.  I use Dura Lube in my twin-cam Harley engine on each oil change (I have also used CD2 brand). These products reduce friction and heat in the engine and you will learn to love this product due to its engine protection.  Just use it in the engine oil, not the transmission and primary.  If you have a bike that uses the same oil for engine, transmission and clutch, Dura Lube will be okay to use.  It may be good for a Harley in the transmission and clutch, but I have not done so and I have not checked with Dura Lube to see if it is okay.  I would assume some in the primary case will do no harm, but the transmission may be a bit risky.  Dura Lube does sell a manual transmission additive that you could use in the transmission.  It would be best to use Harle's synthetic oil in the transmission and primary chain case.  There are many gasoline additives to clean the fuel lines, carburetor or injectors.  Some of them can be too harsh for the rubbers seals in the Harley fuel system.  Check your owner manual or Harley dealer for recommendations.  I have used many over the counter brands with no adverse affects when traveling, because I do not pour the entire contents in the tank, I use only 1/3 or 1/2 of the bottle.  Then I ride off and burn up that tank of gas.  Then in a few days later I will treat again with the second half of the bottle and ride off and burn off that tank of gas.  Do not leave these additives in with the gas overnight.  That gives the additive time to attack rubber seals.  Just pour and go.  Dura Lube also makes gas additives.  It is a good idea to keep your fuel filter, lines and injectors clean, so do use additives at least once each month of riding.  It is no fun being broken down because of a clogged gas line.  Lucas Oil Products, Inc. makes a nice additive for gasoline engines and I find it makes the fuel burn with a louder explosive sound.  It cleans the fuel injectors, piston rings, piston and valve surfaces and is economical as fuel economy increases to the point it way more than pays for the product!  The product also neutralizes poor fuel quality, reduces pre-ignition, loss of compression, burned valves and broken piston rings.  Increases fuel economy, increases power, raises compression, lowers tailpipe emissions, lubes cylinders and adds longevity to engine life.  What more could you want?  I have had no trouble leaving this additive in the tank overnight or for days as it is like an oil-base lubricant.   I believe you will find these products of good use.

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