You wouldn't notice a difference in starting between 10-25%, you are still talking a majority methanol. I think the article left a lot out, but oh well. In some applications people run >60% nitromethane and still manage to light it off.

Look for bright and shiny elements in the glow heads, not dull white or mangled. Also, choose piston/cylinder sets which hold compression best. Weak compression and bad glow heads will be a far bigger problem for Cox than percentage of nitromethane in the fuel.

Glow fuel can become problematic if it absorbs moisture. It will do so simply be not being sealed properly (pressure tight) between uses. Properly stored, it keeps well. Castor can precipitate flakes over time and must be filtered.

  I have run reed valve engines on as little as 5% nitro with no problems, and run them A LOT on 10 and 15% nitro. Nitro doesn't do what most people think it does in model fuel. Just to fly a Cox reed valve engine of any kind you just need fresh fuel, a good plug, and a good health battery and make sure your battery does not exceed 1.5 volts. If the engine has been sitting, the reed may get stuck and will only run on the prime. Force feeding a bit of fuel most often works it loose. Run the engine on the ground for a few tank fulls with the glow clip attached and work the needle up and down so it can flush things out. If it's been sitting for 40 or 50 years, you may want to take the tank off and clean things out. I have seen some strange stuff in them in my time!! Keep in mind that almost EVRERYONE that touched these engine back then was a beginner and had NO idea what they were doing, at ALL !! That can be a double edged sword, meaning that they got tired of messing with it, and just put it away and forgot about it without messing anything up. The other side of the coin, either they or their Dad took things apart to "fix" it and that leaves the door wide open for all sorts of antics!! But the good news is that they can almost always be made right. Just for you own information, Cox regular fuel was 15" nitro and 18% all castor fuel with some U-Con oil added ( not really critical) The red can racing fuel was 30% nitro and same oil specs. If you go to the various Cox engine groups most have photo copies of the work order for mixing fuel back in good old Santa Anna, CA. But if all you can find is 10% nitro that will work fine. Lower nitro will give you just a bit longer engine run because you have to turn the needle in to get to peak RPM. Just run it and play with it. Nothing like actual experience to help your understanding  of what you read elsewhere.
   Type at you later,
    Dan McEntee

Mr Leonard,

Without reading the magazine article, I cannot answer to the author's statements.

I can, however, answer your questions directly.

Yes, fuel can go stale. Keep the bottle closed/sealed for best long term protection. Decant into smaller field bottles so you can keep "the source bottle" closed as much as possible.Start with a bottle of fresh fuel in order to learn your engine(s). Once you have them figured out, check your old fuel by trying it. Trying old fuel before learning these engines is asking to get "kicked in the shins." Trying old fuel once you know these engines is still asking to get kicked in the shins...but if it works, it is cheap!

Here are links for the guy just starting out:

https://brodak.com/fuel/fuel-qt-10-nitro-11-5-castor-11-5-synthetic.html (just for brand/blend familiarity...not what I'd buy)

https://brodak.com/fuel/fuel-qt-15-nitro-11-5-castor-11-5-synthetic-1.html (the lowest nitro I'd go, but as a last resort)

Brodak has a couple more 1/2A offerings (25% Nitro, 35% Nitro), which is what I recommend until you feel comfortable with these engines. These two fuels will be fairly easy starting and run well.

10% nitro is going to make your engines feel anemic,.be difficult to find a consistent needle position (with engine speed hunting), and be a bit harder to start (but not impossible). My Norvels do better on 10% than the Cox engines, but they still do so grudgingly. I suspect this is because the Norvels are .061 and .074 cid engines.
Anything significantly over 35% will show up as hard to needle because the standard Cox needle threads are too coarse for fine tuning for high nitro fuel. Finding fine-thread needles for high-Nitro fuel will be difficult, time-consuming, and not cheap!

CoxEngineForum.com (CEF)...sign up, camp out, and introduce yourself when comfortable. You'll see some familiar names over there. They enjoy the hobby and take it much less seriously over there. The relationship between CEF and Cox International is platonic, not business. You'll see when you go there.

Ok...deep dive if it helps you:
Cox blended different fuels for different performance levels.  Cox even sold 35% Nitro fuel for their engines...and that is pretty hot for Cox engines (but the performance will be sparkling). Yes, you can go hotter...and others have, but I don't recommend going higher than the manufacturer'sr recommendations until you learn these engines.You can find the "original blend formulas" over on CEF.

Glow-plugs:
Cox designed two particular "glow heads" with many slightly different "internal" configurations. Basically, they offered a high-compression glow head and a normal compression glow head. I won't mention the various aftermarket cylinder heads for more commonly available glow-plugs with various heat-ranges, and I recommend staying with the OEM heads (either high or preferably normal compression) until you gain experience with these little jewels. Going to a higher compression sounds like a good idea, but you will be left with using/stacking head gaskets to minimize pre-ignition...and it requires knowing what your fuel and engine/glowhead will tolerate (read that, you need to experiment-a lot). Go visit https://kamtechnik.com once you get comfortable with these engines. He has highly regarded conversion heads for all those other glow-plugs out there.

Nitro:
As normal Cox "Glow Head" convention goes, the higher the nitro, the shorter the lifespan of the element. Going up to 35% Nitro IS within Cox' design parameters. Going as low as 10% is going to maximize the life span of the Glow Head. Staying between 20% and 25% is just fine. Once you learn the engine, you'll like 35% Nitro. However, note the higher the nitro, the faster the engine will turn. The faster the engine turns, the faster it consumes fuel. So I use low Nitro at roughly 22% for most of my general flying and longer flights ("roundie-roundie, up and downie"), and crank up the nitro for less frequent, but more sporty/shorter flights ("roundie-roundie really fast, and anywhere I can point the plane...including the ground-ouch!").

Oil:
Also note that Cox engines are built with a ball and socket between the piston and connecting rod, as opposed to other engines with wrist pins. This construction method means your fuel MUST have castor oil for longer life of the ball and socket. Users have different opinions regarding how much castor oil IS required. Some say a splash of castor in a fuel with all-synthetic oil is enough, but I would prefer to have a known amount of castor...not just a splash/guess.

Parts: http://www.exmodelengines.com,
https://coxengines.ca, and http://www.mecoa.com
All three have an eBay presence too.

My experience:
I buy high-Nitro, low-content synthetic oil mixed fuel, straight methanol, and locally procured castor oil.. Then I use a two-fuel calculator (Thank you, Andy Batts) to mix up my fuel in measured fashion for each engine's requirements. The calculator is just an Excel spreadsheet.
Since I own several 1/2A engines from various manufactures, I mix in small batches. I tend to blend for roughly 18-23% oil (50/50 castor/synthetic for my Cox engines), and adjust methanol input to achieve the desired Nitro content. Most of my 1/2A engines get roughly 23% Nitro just because adding oil and methanol will drop that Nitro content fast. My Norvels like less oil overall (with more synthetic/less castor) and more Nitro...easy enough by mixing in less castor and methanol. The Andy Batts "two fuel calculator" is designed for mixing two fuels with known Nitro/oil content. Yes, methanol-only fuel can be inserted in the calculator as 100% methanol, 0% oil, 0% Nitro. His calculator runs the numbers between two fuels and castor by volume to produce the "new blend."
My larger Fox .35 gets the same treatment, but different numbers. I'll leave that alone since it isn't germaine to this thread.

PM me with your email address...I'll.email you the Excel spreadsheet. In fact, I adjusted his spreadsheet to  prepare smaller batches (8 oz instead of his 16 oz). Experiment with the spreadsheet before mixing live components...or simply buy Brodak.I don't know if Fitz Fuels and Glowplug Boy still operate.
I don't like the idea of mail-order fuel, but this may change as life no longer has me travelling past distant hobby shops. A race engine builder is 30 minutes away...he has methanol in 55 gallon barrels. My last castor oil purchase was via Walmart..and delivered to the local store to beat shipping charges.

I hope this helps!

Dan gave some good advice.   I’ve used 15% nitro with good results.

I don see any mention of tightening the ball joint.   I have found that this helps restore performance.