Is it harmful to mix regular motor oil with synthetic?
Synthetic and conventional will not "mix" and synthetic creates a barrier on moving parts, that conventional cannot pass. So, too much synthetic, as in a 50/50 blend, will be like running an 8 quart system on 4 quarts, without starving the oil pump. It will break the synthetic oil down quicker... in conclusion: The correct combination will give you no problems and will yield the benefits of both conventional and synthetic. With Engine/Motor Oil, the maximum amount of synthetic recommended is 25%. Foaming will develop if more is used, and the foam will counteract with lubrication. Higher-priced synthetic blends have more synthetic and/or more friction modifiers. Synthetic Blend as mentioned earlier is not as cost efficient as adding your own bottle of full synthetic for every 3 quarts of conventional. Just make sure the weight is the same.
Additional info or FYI:
Gear Oils can be combined, but with higher regards to percentage than as with oils. I wouldn't play chemist on my vehicle, but if need be to combine, my professional advice would be that the amount of synthetic combined with gear oil would be as little as 1%, so in other words, if you are not certain what gear oil is in... say, your differentials, drain and refill and do not worry about the remainder of gear oil that is in the differential... or leave the old gear oil (unless burnt) and simply top it off with something like Lucas Oil Conditioner that is recommended for gear boxes and will interact with either conventional or synthetic.
As a whole, greases will not blend/fold unless they are of the same "base" and this is more extreme when in regards to synthetic & conventional. Bearings need to fold the grease and exchange old grease with new grease as the bearings pull the new grease in and push the old grease out. The above mentioned barrier will contain one or the other and not allow the other to fold with it. This is where it is important to know what's what. I'll use u-joints as an example here:
When purchased, if the u-joint is packed with Lithium-based grease from the factory and John Doe's 15-minute Lube pumps the u-joints with moly-based conventional, or worse, silicon-based synthetic...
... even if they say "we flushed out the old grease with the new grease", they will not be able remove 100% of the old grease without actually removing/disassembling/cleaning/repacking/reinstalling the u-joint. So, the lithium-based grease will never be refreshed with new grease; It will eventually burn up around the bearings causing the bearing surface to disintegrate and mix in with the burnt grease, ultimately leading to a more speady premature failure due to metal mixed in with the grease. Anyone who has disassembled a bad u-joint may have seen what resembled the absence of bearings + grease and nothing but graphite powder inside. The graphite powder is* the bearings and the grease was either burnt up or thrown out when the tolerances (or slack/slop) became extreme between the yoke and cup after the bearings became worn.
It could be damaging to an engine to use the wrong API specification oil:
1) Engines that were meticulously engineered/designed using API SH oil (Honda motorcycles for example, but automotive is included) can prevent a higher API rating from sticking to the moving parts in areas such as the main bearings-to-crank journals. If the oil is thrown off the bearing, it is almost as anti-beneficial as no oil at all.
2) When the vehicle has a "flat tappet" style valve train. Alkyl-zinc, now replaced with the anagram "ZDP" or "ZDDP", is needed as a cushion between the camshaft and lifter. SM rated oils are only compatible with API MIL-specs for SJ & SL. Additives such as STP Oil Treatment for 4-cylinders (more ZDP than for 8-cylinders) has a lower ZDP content than the pre-1996 formula. Special Racing Oils or Motorcycle Oils commonly contain more ZDP to protect main bearings and valve train, as well as additives for break-in periods, which commonly contain so much ZDP that they are recommended to be drained at the first scheduled oil change. The break-in additives can be used in part, to give the .1% (1000ppm)- .16% (1600ppm) content as specified in your owner's manual. Too much for too long can cause sludge, so the 41600ppm+ in say 1pt of the additive, even if reduced by means of diluting with 8 quarts of oil, resulting in a content of a 2600ppm. ZDP/ZDDP was lowered by the SAE/API to extend the life of catalytic converters. If your vehicle did not come equipped with one, the need for lowed ZDP/ZDDP does not apply to your vehicle and the need for more ZDP/ZDDP most likely does... but will not hurt anything otherwise.
You will have no issues with mixing regular motor oil and synthetics together. As for longer drain times peope please use your heads. If you are not going to change the oil as often please change your filter around 3k they still get as dirty as they did with regular oil.
break the oil down and eat away at the aluminum and steel inside
the engine, but even with synthetic oil, if you wait too long to change the oil it will also break down, and have more base stock and less viscosity modifiers. When synthetics wear out, become acidic, and eventually become saturated with suspended soot particles, just like regular oil, it can also harm your engines interior parts. Again, an oil analysis is a good investment to determine the optimum oil change interval. Never exceed the manufacturer requirements for normal service. You probably could go a bit longer between oil changes with a synthetic, i.e. following the normal service schedule even if you fall into the severe service category, but that is not advised. In short it may give you the peace of mind that it's
Most manufacturers of synthetic oil advise users to not exceed the manufacturer's recommended oil change interval. Part of this is self interest (they don't want to be liable for any damage) but the real reason is that synthetic oil, while it does have certain advantages, still becomes contaminated.
The 3000 Mile Myth
The 3000 mile oil change interval has been pounded into people's heads for decades. It had a scientific basis when engines used non-multi-weight, non-detergent oil. It no longer has any scientific basis, but it is still being promoted by certain entities.
The 3000 Mile Fact
There are some vehicles that need 3,000 mile oil changes, but it's not because the oil goes bad after 3,000 miles. One example is the Saturn S series. These vehicles have a timing chain system that is very sensitive to clean oil because oil pressure is used as hydraulic fluid to ratchet up the timing chain tensioner. If varnish forms in the timing chain tensioner bore then this system can fail and the chain will become loose and eventually break. I bought a used Saturn and had this happen only one month to the day after I bought it. Dealers have gone as far as tearing out the normal service schedule (6000 miles) and leaving only the severe service schedule. If your engine is destroyed (under warranty) by a failed timing chain then the dealer will legitimately request evidence of oil changes. Unfortunately this problem usually won't manifest itself during the warranty period.
3000 Mile fact
The 3000 mile oil change interval can be extended with today's oil and fuel injection technology. If you have a normally aspirated engine you need to change oil more frequently due to the lack of efficiency of carburetors, they can run excessively rich at times causing fuel dilution to your oil with limits it's lubrication ability.
As for blended synthetics, they have approximately 3% to 5% of real synthetic in them, not a good buy for the price.
Synthetic blend motor oil will contain 25% synthetic base, and 75% conventional base. This is due to the API regulations that allow up to 25% substitution of synthetic into a certified conventional oil without retesting. The additive package of the blended oil is still the same as the conventional oil, due to the same API regs. Synthetic oils will generally use a different additive package than conventional oils, so its a bit hard to say how mixing conventional and synthetic oils together will turn out. However, there are a number of times I did just that on my vehicle, and the engine is still running fine with over 220k on it. I don't do that anymore, however, as it doesn't save that much money. In the end, mixing the two types is not a recommended way of doing things, and purchasing straight synthetic doesn't cost much more. Now if you are low on oil and need to add a quart or two to get home, put in what you have on hand rather than run low. Then change at your soonest convenience.
Synthetic oils are manufactured from standard crude oil, they are simply hydrocracked to eliminate all impurities. The mixing of natural oil and synthetic oil at any ratio does not have any negative effect except adding the impurities of conventional oil to the pure synthetic. It is a common myth that you can't mix the two, but it cannot be farther from the truth.