Sir, am also a once a year synthetic oil change practioner. My two gasoline fed cars are 9 and 6 yo with a 7-8K km annual usage and they were both inspected recently via a general oil pan cleaning and a top cylinder cover opening..Both shows a very clean pan and innerwalls of the blocks without any traces of gums deposits. Just monitor your level lang and change it within 12months..to me its sulit na. My premise of not changing it based on milage or months (time) based whichever comes first is that in this way I could contribute of not wasting the Oils, money, etc..lets not contribute to the unecessary wastage just for the sake of being OC in sa car maintenance. sometimes we have to check the Facts and not just by hearsays and others experiences..If I could only get a good quality Oil Filters then I would have it extended more than a year pa.

Caveat!!! just because this works for me will also works for other. Have a good read about Lube oils and from there you can decide for yourself.