I change my oil every 4500-5000 miles (for me that is between 7200 to 8000 Km).
I change my primary oil every other oil change.
And The tranny only every 2 years (generally every 20K) only because I have shotgun shocks and have no access to the tranny bolt so this one my Mechanic does when we change tires.

I have 67K so far and no complaints.

 

3,000 engine and 6,000 trans and primary. I don't buy waiting to 5,000. Oils have improved but can get just as dirty.

 

And that used synthetic oil? I keep it clean in the draining process (suck it out with a vacuum pump siphon) and pour it into the fuel tanks on my diesels to mix with the #2 fuel - eases the wear on the pumps and injectors attendant to the low sulfur fuel we now are obliged to buy... (Mostly, depending on need, I use 30# bar oil for this too.)

I am curious about how many add top oil and/or stabilizer to their gasoline? (I do, fyi)

 

3000 in engine, like evryone else said short trips and cold weather puts water in the oil. hot weather and hard riding will shorten the additive package in the oil.