Can you mix full synthetic with semi synthetic oil?

Answer 1

Yes because semi-synthetic motor oil is a blended mixture generally 70% regular motor oil mixed with 30% full-synthetic oil therefore by adding or blending more full-synthetic oil into the mixture merely increases the percentage amount of full-synthetic oil within a pre-blended semi-synthetic brand.

However it should be noted a 2-year motor oil test done on 75 New York taxi cabs in 1996 showed no significant nor any discernable differences whatsoever between using regular motor oil with a package of additives versus synthetic motor oil in terms of performance, sludge buildup, and wear and tear. It's a tie or in other words it looks to be more of a matter of financial capabilities whether a motorist can afford the overly expensive price of synthetic oil mostly for the peace of mind since apparently synthetic oil is practically no better and no worse than regular motor oil with additives

On the other hand if the original question also asked, "Which is better, synthetic or semi synthetic or regular motor oil whether with or without additives?" then the answers depends upon certain circumstances.

Synthetic motor oil is thinner than regular motor oil but withstands high and low temperatures better and therefore recommended for high performance racing engines or for use in very extreme hot and/or very extreme cold climates. Synthetic oil also takes longer to breakdown which translates into slightly longer periods between oil changes although still requires periodic oil changes. Full-synthetic oil is recommended for cold climate start-ups because its thinness makes it less sticky and less tacky between the moving metal parts of a cold engine which makes it easier for cold engines to turn-over compared to using regular motor oil which is a thicker oil. But synthetic oil is warned against using during a new engine's break-in period and also warned against using in engines prone to leakage because the thinness of the synthetic oil increases leakage particularly in older engines.

Semi-synthetic motor oil consists roughly one-third synthetic and two-thirds regular motor oil and said to offer many same exact advantages as full-synthetic but at around half the price or so.

Although regular motor oil with an additive may be cheaper it apparently works just as good as the more expensive synthetic oils and not worse nor better but essentially the same in regards to lubricating and protecting engines.

For most motorists, although synthetic motor oil is slightly better than regular motor oil it's probably not likely worth the overly expensive price tag unless you're into high performance racing or live in very extreme climate temperatures otherwise everything else about full-synthetic motor oil sounds to be pure hype or else we would've expected any noticeable degree and/or some kind of significant difference in at least one engine out of the 75 city cabs from a 2-year motor oil test instead of zero difference, wouldn't we?

The motor oil used in the Consumer Report 2-year motor oil test used a full package of additives but no tests were done using just plain motor oil without the additional additives, however, an oil expert strongly advises and warns against using any additives in article entitled "Motor Oil Myths and Facts", quote: "Do not use any oil additives no matter how much they are hyped on TV. They provide no benefit and can interfere and react with the additives already present in the oil. Some additives have particles that can clog oil passages and clog filters. Common additives that are heavily hyped are Slick 50, Duralube, and Prolong." Unquote.

As of 2009 in comparison to 30, 20, or 10 years ago, the quality now-a-days of standard plain motor oil usually already contains its own detergent additives and is improved to where it now appears there's no longer any appreciable difference in the performance or benefit between using either plain regular motor oil without additional packages of additives vs synthetic vs semi-synthetic as long as periodic oil changes are maintained as they all work equally the same and equally well at performing the functions they're designed to do. In other words as long as a shop mechanic performs reasonable periodic oil changes over the next 10 or more years then any car owner who believed their car had always been using expensive synthetic oils even though the mechanic had been using cheaper plain motor oil whether with or without additional additives, the owner would not be able to distinguish any difference upon mechanical inspection of the engine parts.

In conclusion and if money is of no concern then the only lazy advantage to using synthetics versus regular with additives appears to be the slightly longer periods between periodic maintenance oil-changes. I haven't researched any actual 10-year span statistics although I'm only guessing it could perhaps be something like a difference between maybe 3 synthetic oil-changes versus maybe 4 regular oil-changes per decade?