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.