[quote author=S1lverstreek link=topic=135723.msg2925724#msg2925724 date=1234240645]
...just check it every time you fill it up at the pump to see how clean/dirty it looks. If you know what dirty oil looks like, you'll know when to change it :wink:
Just an FYI. The color of an oil means nothing. Some add packs in oils darken quickly, some don't. For example, Mobil1 gets dark very quickly but it can handle long intervals.
The best way to verify the life of the oil is to have it analyzed, as bustednutz indicated above.
By the way, based on UOA's, RP isn't that great of an oil in our engines. If you're going to go synthetic, Pennzoil Platinum has shown and proven to be a fantastic synthetic oil, especially for long OCI's.