Nissan Z24 engine, 4 quart capacity, in an 89 pickup. Manual specs 10w-30 or 10w-40. It has had regular 3K miles changes since new, never had an engine problem, never been opened. Castrol GTX 10w-40 is the only oil it's ever known. Nissan filters only. It now has 96K miles on it and is driven much less than it used to be. In that last 9 months, it's regular service amounts to only about 500 miles per quarter (3 months). One thing that I've learned here is to start considering oil changes based on time, for engines that don't always get warmed up and don't see much regular driving. Another thing is the potential for sludging under such conditions. I'm now considering using 3 quarts of the GTX and adding a quart of the 0w-30 German oil to top it off. I'm thinking of the cleaning potential of the synthetic under the low-usage conditions. What do you think? Am I fixing something that isn't broken? Truck runs fine. Has never used oil.