I wouldn't say its not recommended. Conventional and synthetic both meet the same application standard and would both be compatible in an engine requiring such. One of the biggest difference is the flow characteristics of them. They both are asked to lubricate the engine, but the molecules in a synthetic are more engineered or refined from their crude oil partner in conventional oil. If the lubrication molecules were a ball bearing, synthetics would be a much smoother ball bearing that rolls and flows more easily. This ability to fllow more easily and smoothly is the great benefit to synthetics, but in an old engine it can cause some challenges. One example is that it will flow more easily out of seals and gaskets. You should also keep an eye on the pressure difference. I have an older truck and put a full synthetic in it, the end result was more leaks out of the engine. Hope this helps. Good luck.
This question comes up a lot from people who've just bought a used vehicle and are wanting to start their history with the car on fresh oil. The short answer: generally speaking, not any more. The caveat is that your engine must be in good working order and not be leaking right now. If that's the case, most modern oils are fully compatible with the elastomeric materials that engine seals are made from, and you shouldn't have any issues with leaks. The longer answer:
Mixing Mineral and Synthetic oils - the old and busted concepts
For the longest time, I had this to say about mixing mineral and synthetic oils:
* If you've been driving around with mineral oil in your engine for years, don't switch to synthetic oil without preparation. Synthetic oils have been known to dislodge the baked-on deposits from mineral oils and leave them floating around your engine - not good. It's wise to use a flushing oil first (see below).
* If you do decide to change, only go up the scale. If you've been running around on synthetic, don't change down to a mineral-based oil - your engine might not be able to cope with the degradation in lubrication. Consequently, if you've been using mineral oil, try a semi or a full synthetic oil. By degradation, I'm speaking of the wear tolerances that an engine develops based on the oil that it's using. Thicker mineral oils mean thicker layers of oil coating the moving parts (by microns though). Switching to a thinner synthetic oil can cause piston rings to leak and in some very rare cases, piston slap or crank vibration.
* Gaskets and seals! With the makeup of synthetic oils being different from mineral oils, mineral-oil-soaked gaskets and seals have been known to leak when exposed to synthetic oils. Perhaps not that common an occurrence, but worth bearing in mind nevertheless.
Mixing Mineral and Synthetic oils - the new hotness
That's the thing with progress - stuff becomes out-of-date. Fortunately for you, dear reader, the web is a great place to keep things up-to-date, so here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website.
There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synthetics. Polyglycols don't mix with normal mineral oils.)
There is also no problem with switching back and forth between synthetic and mineral based oils. In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.
There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - your seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products..***:D
I am not certain how old of cars you are talking about, but synthetic oil was considered satisfactory for cars in the early 1970's. Synthetic oil has the same viscosity ratings as regular oil. If an engine is worn out, then that is a different matter. Thinner oils, whether synthetic or regular oil, in a worn out engine may suffer from low oil pressure, and may burn or leak the oil quicker, making a heavier weight oil a better choice. Synthetic oil, at its higher cost, becomes uneconomical, in an engine that goes through oil quickly.
Older vehicles have different material making up the original seals. Most are not compatible with the chemicals in synthetic oil. You'll wind up having oil leaks everywhere.
Synthetic and semi synthetics come in weights and int summer/winter viscosity grades just like regular, mineral oil motor oils. For example, my 28 year old pickup has used 10W30 o4 !0W40 since around 50k miles. It still runs and operates, all conditons and speed limits like it did when new.
What you've heard is absolutely wrong. Read these articles about synthetic oil.
http://ricksfreeautorepairadvice.com/syn...
http://ricksfreeautorepairadvice.com/wha...
you are hearing myths that will just not go away
I heard a few people say it's no good for old cars because its a thin oil, older cars need thicker oil to fill the void of the worn parts or something of that sort. Synthetic oil
