well we used to change oil in all our rides at 1200 KM interval till 2008 since most oils were mineral at the time.
once we started getting synthetic oils (pioneered mainly by ZIC in pakistan initially) i gradually increased change interval to 3K and now m using Total 9000 and Taglon 5w-50 and change interval is 5K in all rides.
I just have a feel with oil, as per my experience in summers specially once the oil crosses 4K kms the radiator fans usually turn on and off more frequently suggesting that the cooling capacity of the oil has reduced.
considering not only the environment but also the low grade fuel that we get in Pakistan i dont recommend stretching any oil beyond 5K / 6 months interval. This is to ensure that you will not have to get anything related to engine over haul till 300K KM.
by the way i recently sold my ride which crossed 300K Kms (never any engine issues) and have another one running AL HAMD U LILLAH fine at 325k KMs.
link to my ride page