Confused about changing from conventional to synthetic

The biggest problem I've seen is leaking when switching to synthetic oil, and that stops shortly after switching back. It's the detergents, seal conditioners, and anti-foaming additives that are not compatible but that includes between some brands of regular oil.

There is little to be gained by using synthetic oil. The biggest benefit is it flows better at cold temperatures. All oil has two ratings on the container that you should look at. You'll see one that starts with an "S" for "spark ignition" engines. When a significant improvement is made, the second letter advances to the next letter in the alphabet. The last time I looked, which was quite a while ago, we were up to "SG". I suspect by now we're at "SH" or "SJ". Given the age of your car, any oil you find in any store will more than meet the needs of your engine. The only time you have to be concerned that an oil is lacking in some property for your engine is if you find a container in someone's garage and it has been there for years.

The second rating is similar but it starts with a "C" for "compression ignition", meaning diesel engines.

I should clarify too, that "SG" rating applies to all refiners. It doesn't get a new rating when one refiner thinks they improved their product over a competitor's. The entire industry must follow the guidelines that specifies the various characteristic of the oil, then all manufacturers must meet those specifications. In other words, when new requirements are established, it is up to each refiner to do whatever is necessary to meet them, then they are allowed to use the new rating letters. You'll find everyone's oil meets the new ratings at close to the same time, so as long as you buy new oil as you need it, you'll always have the right formulation, or better, even for very new cars.

5W30 oil is pretty thin already and it's going to flow well enough in cold weather. All manufacturers have gone to those light oils for lower friction and resistance in the engine. That is supposed to improve fuel mileage. If you think you need to improve on what the engineers decided is acceptable, use the synthetic, but stick with it. Don't try to combine some custom formulation because you'll have non-compatible additives fighting each other. You'll likely find its additives, particularly detergents, attack the seal conditioners in the old oil, but over time the seal conditioner additives in the synthetic oil will take over. High-mileage engines is usually where you'll find leaks showing up when switching to synthetic oil. I would assume that also applies to the oil sneaking past the piston rings and being burned. That likely won't be bad enough to cause smoke, but you'll see the black soot inside the tail pipe.

When it comes to oil, I'm an expert at reading labels and service manuals. For other things I defer to the other experts and reading about others' experiences. I have no reason to argue with the person you spoke with. My best suggestion is you're trying to go to something that has benefits that your engine doesn't need or may be of little value. That's why I spent so much time explaining the oil ratings. Those ratings tell me that today's regular oil is a lot better than what was available when your engine was built. I don't want you to spend your money on more expensive oil if it isn't going to be a good value.

All oils have seal conditioners and other additives. Basically, oil is oil. It's what they take out of the crude oil and those additives they put in that are constantly being improved. That's when they get the newest "S" and "C" ratings.

One of the original selling points of synthetic oil was that it lasted longer so you could drive further between oil changes, but you have to remember my famous quote, "oil is oil". The high temperatures cause it to break down and gradually lose its lubricating properties. Some of the additives delay that from happening. The bigger concern is the stuff that builds up in it. Besides lubrication and carrying away heat, oil is supposed to pick up sludge from blowby and carry it to the oil filter. The filter has to be porous enough for all the oil to get through to meet the engine's needs. That means a lot of the suspended solids can make it through too. We change oil regularly to remove those solids and to replenish the additives. If the manufacturer of synthetic oil says you don't have to change it as often, that's because the additives and the lubricating properties are still sufficient to meet the needs of the engine, but what about the suspended solids that are circulating? You want to get those out too.

Also be aware that when synthetic oil first became common in the mid '70s, it was advertised as lasting 15,000 miles! You know it has gotten better since then, so why did they drop that recommended interval down to as much as 5,000 miles? I think they found the hype was more optimistic than what they could deliver.

To get back to your question on seal conditioners, as I mentioned, all oil has them. The problem is there are a number of different chemicals they can use for the purpose, but they aren't compatible with each other or with some of the other additives. For example, one oil might have a chemical for a seal conditioner that is attacked and rendered ineffective by the detergent in a different oil. THAT'S where the incompatibility can come in.

Without analyzing in-depth what you're thinking about the viscosity, let me simply explain it. A multi-viscosity oil like 10W-40 does not change its thickness as its temperature changes, as many people think. There are standardized tests where they run an oil through a specifically-sized orifice at a specific temperature, and they watch how much flows in a specific amount of time. Multi-weight oil is tested at two or more different temperatures. 10 weight oil will run through at a certain rate. 40 weight oil also runs through at a specific rate. At the cold test temperature, 10W-40 will run through at the same rate as straight 10W oil. At the high test temperature, that oil will run through at the same rate as 40W oil. A better way of thinking of it is a multi-weight oil will become less thinner at higher temperatures. It acts like 10W oil when it's cold, and it acts like 40W oil when it's warm.

You don't need to add anything to your oil like seal conditioners or detergents. Everything your engine needs and then some is already in it, regardless of which type you use.

Let me add one more dimension to this sad story. The engine in my '88 Grand Caravan daily driver has well over 400,000 miles on it. As an experiment for my students, I have not changed the oil or even removed the drain plug in over 13 years and 100,000 miles. I need to add a quart about every 1,000 miles. We were taught that there is enough additives in one quart of oil to meet the needs of the engine, so in fact, I'm adding fresh additives three times more often than recommended. I do replace the filter when the mood strikes me, like once every two years, because if it gets plugged, the oil will bypass it and stop getting filtered. Please understand that this is not neglect. This is abuse, and I'm not suggesting anyone do this. I did it to prove a point about what some engines are capable of. Also, I have only used the cheapest oil I can find at a local farm and home store, and I have never changed brands. I have an '80 Plymouth Volare that has only seen Valvoline oil in its entire life. I'm not advertising for it or promoting it. That's what was cheapest when I bought that car in 1980. I have a '93 Dynasty that I've used nothing but Mopar oil. I change that oil every 2,500 miles, ... And it has had exactly one oil change in its life so far. I might make it to 5,000 miles this summer once I get it out of storage! Now, you know Chrysler is not in the oil business. Just because they have their name on the container doesn't mean much, but you know they won't buy cheap oil and risk their reputation.

To get back to your salespeople and some of their comments, if you do want to use synthetic oil, you had better not have to add another can of anything. If anyone tells you to add a can of seal conditioner, ask them why it isn't in the oil already. If this is supposed to be a stop-gap measure until the new oil's additives have time to take effect, just stick with that new oil at every oil change and it will eventually do the same thing. One of my students had an old Ford truck that didn't use any oil. He too wanted to switch to synthetic. Within a few days he had a leak from the rear main seal that went from a small spot where he parked within a couple of hours, to a puddle the size of a gallon paint can within half an hour after about a week. He switched back to regular oil and most of the leak dried up but it took over a month. After that the rear of the oil pan was always wet but there usually was no spot on the ground where he parked. Would that major leak have dried up if he had stayed with synthetic oil? We don't know because he wasn't going to wait and make a mess everywhere to find out.

For my final comment of great value, we used to see people selling synthetic oil at old car shows, and they always had a display where they had opened cans of various brands of oil in dry ice, then they had paint sticks in them that you were supposed to try to pull out. Some oil was so thick at that cold temperature that you pulled the entire can right out of the display. Most of the oils looked like really thick honey. Of course the stick came right out of the synthetic oil. That stuff ran like regular oil when it was warm. Now that I think about that, how can that be compared to a multi-viscosity oil if it is so much thinner than 10W oil? It seems like two different fluids are being compared. The propaganda says your engine will start easier in cold weather because the oil flows easier. But the days are long gone when people used to drain the oil to warm it on the stove in winter so the engine would start when it was cranked by hand! My '80 Volare always started down to at least 30 below, and the oil pressure came right up to normal. What needs improvement over that? Cars today with electronic fuel injection start easier, and they use tiny starters with gear-reduction drives that work just fine. You won't find a car anywhere today that will start only with synthetic oil but not with regular oil. That takes care of the "easier starting" claims.

There is a case to be made too that synthetic oil flows better at cold start-up so it provides better lubrication. That sounds good at first until you realize that we need the oil to flow because it runs out of the gaps in the critical engine bearings and has to be replenished. It is possible to do permanent damage to those bearings if they run without oil pressure for as little as a few seconds. Well, if regular oil doesn't flow as easily INTO the bearings when it's cold, it won't drain OUT of them as fast either.