I own a 1986 Chevy K10 with a newer crate 350 and I typically run 10w-30 conventional motor oil. I live in the Northern USA and only drive the truck over the summer. During the winter months it is in storage but I do start it every 2 - 3 week and let it run just to get some fuel through it. I was once told that synthetic oil can damage the seals and cause oil leaks and in particular can cause the rear main seal to leak. I've also heard that this was an issue with early synthetics but with advancements in technology that synthetics have improved and this is no longer the case. What is your recommendation?
If you still had the original engine then yea its best to prob stick to conventional. But since you have that crate motor in there might as well run synthetic. You'll get longer life out of that crate motor and it would make starting it up in the winter much easier on the engine and if ever noticed any small leaks starting and even if you don't just hit the engine with a a half a bottle of AT-205 and that stuff works miracles on those rubber seals and will keep that crate motor in tip top shape. One thing too is drive the car a bit when you start it during the winter. Cars don't do great sitting for a long time so even a very short trip just the engine warm up and stuff would be better than just letting it stay parked and go through fuel especially with the crate combo in the old chassy.
Check viscosity requirements... I think you'd be better served by 5-30 in your location... check that specific engine info tho.
Syn always!
Here's the deal with old seals and leaking. Older conventional motor oils has esters (hydrocarbon molecules) in them. these esters would leach into old seals/gaskets by design so they would swell and prevent leaks. This dates way back to cork gasket days and stayed in place for a long time! So replacing the conv oil that had esters with Syn that doesn't, the esters in the gasket would leach back into the oil from the gasket and thus leaving voids (shrinkage) and leakage. Easily solve (rarely needed anymore tho) buy just using 1 quart of conv oil with the syn. Just too many advantages to using syn to not use it.
Definitely use synthetic oil. Way back in the 1980s when synthetic was first produced, there were some seal leakage problems due to the formulations being used in syn oil. But that's all been solved, and the advantages of synthetic oil are so great that there's no good reason not to use it. It performs very well in climatic extremes, and doesn't need changing as often.
Less than 80-120k miles (depending on usage factors), pure synthetic in 5W30 (winter/cold), pure synthetic 10W30 in warmer conditions.
Higher mileage I'd use synthetic blend in the weights listed above.
Also, power blast and inspect your frame for rust/damage. Very poor corrosion protection in 86, it can fail and cause you lots of grief.
Thanks for the AT-205 tip. I've run 1/2 quart of Lucas in the past which did seam to help but didn't totally fix the problem. With AT-205, I assume you would wait until your engine oil dip stick is a quart low and then substitute the AT-205? I know the 350 isn't the original engine as the VIN on the truck says it originally came with a 305 so I am assuming it is a crate as it is in pretty good condition yet. I'm about ready to store the truck for the Winter but when next Spring comes, I'll run 4 quarts synthetic and 1 quart of conventional 5w-30. If I run into any rear main leaks, I'll then give the AT-205 a try! Many thanks for the quick responses, this is a great resource!