Trouble with synthetic is it is so slippery it can find it's way out through worn seals and gaskets where ordinary oil does not. It's unlikely you will get 15,000 miles without adding oil every 1,500-2,500 miles so at the end, the original oil has been replaced. Then there is the accumulated carbon from a carbureted engine. By 5K the oil will be black. Keep us updated on this, I'm curious.
Hi Mike,my Jasper engine has 60,000 miles on it and I have never had to add oil to it.I use a 160 degree thermostat to keep it cool,it doesn't get cold here so my heater keeps it warm when I need it.My seals and gaskets are good.The valve cover doesn't leak.I had my Jasper engine installed in 2009,it's a remanufactured 2.4,I use a Weber which is not old.The remanufactured Transmission was put in a few months ago,he said he put synthetic fluid in it.It doesn't leak either,I have a 1 year warranty on it.I was using Castrol 10W30 before this oil change,was using the blue Nissan filter and last 2 were Wix's.It will be awhile before I get 1500 miles on this new oil,but I will let you know how it's doing and if it gets low on the dipstick,I put 4.25 quarts in it.It will be next year before I get 5000 miles on it.I just go to work and back mainly,am a homebody.I hope this new oil does good.My Nissan is my best friend,never lets me down.
Hi ya'll,got a little over 1100 miles on my Mobil 1 Extended Performance oil since I change it in August 5,2017,The dipstick is reading full,no leaks,Spark plugs are burning clean.Oil look's good on dipstick.It idles great,running like a Dream.Also my clutch is doing great since I installed a new clutch master cylinder,fluid is yellow.Will keep ya'll updated as I put more miles on it,just go to work and back.Got a little over 61,000 miles on Jasper engine.If you can afford one,get it.
What worries me is that in 15,000 miles you are going to absorb a lot of combustion chamber deposits and condensed fuel contaminants into your oil, diluting it. The oil may be fine but what's in the oil may not. In addition, no engine is going to run that long without using some oil so you will need to top it up. I imagine 2 to 3 liters at the least which is almost an oil change. Id be happy getting 2K out of a liter.
EFI engines run cleaner. Carb engines don't and why the oil should be changed more often.