You can switch to a good synthetic oil if you want to. They are proven to add longevity to engines. This all comes down to driving conditions for the most part. If you drive in dusty conditions, you need to do your oil and filter changes more frequently. On a cast iron block like you have, conventional oil is what these engines were designed to run. The real key here is oil changes every 3 months, or 3000 miles with conventional oil, and every 5 months, or 5000 miles with synthetic oil, and always change the filter with every oil change. If you live on a gravel road, or travel them often, then you need to change your oil more often. As for additives adding longitivity to your engine when there are no issues, no, they wont. Clean oil and filter is the key to making your engine last a long time. My 97 Silverado (5.7L) has almost 250,000 miles on the original engine, and it runs excellent. There are people with your same engine with well over 400,000 miles on the original engine. There are even a few with over 1,000,000 miles although that is rear.