Oil Change Interval

[Cabreco]

How often should I change my oil?

This is currently the status of my Fiona. she has 25,000 original miles.

When I bought it May 2011 I added seafoam to the original oil that was in it. I ran it for 10 minutes then drained it & filled with oil, changed the filter added seafoam again to the crankcase ran it for about 100 miles & drained it & replaced the filter again. After that she was really clean. I changed the oil & filter.

Last May, I did an oil change/filter again with Rotella T 15w 40 oil. Now since the oil change I have put about 2000 miles on her in the last year. She never sits idle I ride her at least a few miles 3 or 4 times a week. I know the manual says annually but then oil technology has change signifiacantly in 30 years.

So this being the case, what frequency should I use to change the oil? or should I use mileage?  

Now if I use Synthetic Rotella would the change interval change?

[sgtslag]

There are variables, but the answer is:  "It depends..."

If you want to know for certain, do an oil analysis.  What is an oil analysis?  Here is Blackstone's explanation of just what they do:  link.

The 3,000-mile car rule (was 1,500 miles for bikes), has not been valid, for more than a decade; newer cars/trucks have had 5,000-7,000 oil change intervals recommended by the manufacturers since around the year 2000.  However, M/C transmissions still tend to rip viscosity improving (VI) additives, found within multi-viscosity conventional oils, apart after 1,500 miles.  Full synthetic oils do not have VI additives in them, so they retain their full, multi-viscosity properties, longer.  Shell T6 is not a true synthetic, being made from Class III base oils, but it sure acts like a Class IV, full synthetic oil.

You can safely change your oil anytime, but it will last 3k-7k miles, if full synthetic, without analysis.  If it is conventional, 1,500 is safe, but longer will need analysis to know when it is not safe.  Sorry, but there is no hard answer.  Cheers!