I have a copy of the Royal geographical Society 'hints to Travellers' for 1938. It has a couple of pages of information on paraffin stoves, including the remark that roarer burners are useless over 18000 feet, and the rather startling suggestion of using a paraffin/petrol mixture at high altitudes.

Apparently before the 1936 Mount Everest Expedition Primus Stoves were tested at RAF Farnborough in a decompression chamber up to the equivalent of 40000 feet. The results were in brief, that a No 5 Primus Burner worked fine up to 34000 feet but above that altitude special nipples and caps were required. Does anyone have any more details about this or about Messrs. Condrup Ltd., 77 Fore Street, London EC2 who made the nipples and caps.

There is an article on Condrup on the site. I have seen these claims re the silent modified burners before & I think I may have some info in one of my old books somewhere - I'll take a look.

John Hunt's book mentions that the primuses used were adjustable self-pricking silent burners modified with with oversized spirit cups tested to 40000 feet in a decrompression chamber - maintained by 'Ed Hillary' - an illustrious fettler...

This is not so strange a proposition. The general caution about mixing paraffin and petrol is a sound one for reasons of safety, because certainly a classic keroburner cannot safely burn petrol, because of the fact that there is no shutoff, and pressure release will release volatile petrol fumes that can forthwith ignite.
That said, in the hands of the experienced, a mix can be safe and useful, particularly in cold weather. We've discussed this here on the board before. Ten to fifteen percent petrol to 85% I've experimented with. In my experience, it's safe and increases ease of lighting in harsh circumstances.
But make your own trials and judgments, and don't rely on me. And I myself wouldn't go above those percentages for petrol.

I should have said 10-15% petrol to 85-90% kerosene, lest someone think I meant up to 85% petrol!
The rule here is that of reason. The general caveat has to be: don't use petrol in a keroburner. Actually, it burns fine, but shutting it off is the big problem. And the stove also could overheat, but lacks the Swedish handgrenade feature which makes the small classic petrolburners so charming, but which in a keroburner could create a really big problem.
But a small percentage of petrol presents no safety problem, and increases the volatility of the fuel slightly but usefully for cold conditions.
But you know how it is, especially with us fettlers and self-reliant mountain men. If a little is good, more is better, etc., 'til you've created a problem for yourself. Thus the general rule must still stand as a necessary caution.