I've done a search on the search page and I've looked all over the forum in the past weeks. I'm still confused. Is it safe to run a synthetic or blend in a higher mileage motor. Facts are below. 1. Nissan Z24 (2.4 liter four cylinder, cross flow heads, 8 spark plug design.) 2. 155,000 miles 3. Does not burn an abnormal amount of oil. Drove from Idaho to Kansas and used less than a quart. 4. Shows no sign of any leakage except at the drain plug where its always had a leak due to the fact that I've been to lazy to replace the O-ring on the plug bolt. 5. Truck has ALWAYS had regular changes every 3000 miles with different brands of dino oil. Quaker state for its first 10 years of life after that whatever I felt like putting in. I'd like to try a synthetic blend or a full synthetic but I don't want to create an oil leak.