The first step is draining the oil. (Very important), this must be done even before you start the engine. When Pit bikes come from the manufacturer or I should really say the engine manufacture, they are tested to see if all the mechanical parts work but the manufacturer does not run it in. From the journey overseas to your country the engine manufactures put in a substitution engine oil just for this journey. This oil’s only purpose is the absorb moisture comes from the sea which is built up in over the duration of the journey. When you drain the oil by removing the sump bolt (usually a 17mm nut) if you look closely the color of the oil won’t be that dark in fact it looks like cooking oil. It will contain a lot of bubbles which is water trapped in the oil. This is the moisture from the sea captured by the oil.
The second important thing you will notice is the oil contains small engine shavings. These metal shaving come from the engine itself. It’hasn’t been run in and the engine still has rough particles that have broken off and absorbed by the oil.
By draining the oil you remove the weaker moister capturing oil along with the metal shavings. These shaving however minute can ruin your engine internals over time. replace the engine oil with a branded engine oil. Motor bike engine oil 15w-40w for 4 stroke bikes trusted brands like Shell, Valvoline or Silkolene. You will need to add about 850ml-900ml (less than a liter) for 125cc engines, a little more for 140cc or 150cc oil cooled engines.
After replacing the old oil with the new oil, go through the running in process which is ride the bike for two tanks of fuel at half throttle only. Strictly only use up to second gear for this stage then open the throttle after run in and go hard.