I seem to remember that there was a problem with some of the multigrade/synthetic oils if you changed to them after running the engine on non multigrade for some time. IIRC it was caused by lead build up with the std oil. The lead was then removed by the multigrade which gave problems with oilways blocking as so much of it was dumped into the oil.
If you ran the engine on multigrade from day one, then the lead didn't build up and there wasn't a problem.
Lots of misconceptions on oil. The subject is preheat, of course, but let's tackle oil.
The standard misconception is that mineral oil leaves varnish, which is then removed by the use of synthetic oil. The varnish theoretically ends up potentially plugging galleys, etc. There's no lead buildup.
Another popular misconception is that there's mineral oil, AD oil, and multiweight oil. They're all mineral oil, and they're all compatible. That is to say, straight weight oil is mineral oil, as is multi weight.
There's no issue with running alternating straight weight and multigrade oil. Traditionally, straight weight oil has been used for the break-in with a new cylinder, top overhaul, overhaul, or new engine, followed by the operator's choice of oil (often multigrade). I've worked for a number of operators who preferred straight weight in the summer, and multi-grade in the winter.
A brief period of time involving a series of lawsuits against Phillips for their XC-II oil caused some disharmony in the community and displeasure regarding multigrade oil. Current XC oil works for both break-in, and regular every day ongoing operation. Switching between this and straight weight won't cause contamination or varnish issues.
Another common misconception regarding oil is the change interval. Some feel it should be based on hours alone, but it should be by calendar interval as well as hours. An airplane which doesn't fly all year still needs regular oil changes. An airplane which flies regularly needs regular oil changes, too. It's a whichever-comes-first proposition.
Pprune is mostly a UK/European forum. The UK is rarely -6C or -12C.
If it is, which is extremely rare, prob99 you cannot get to the airport.
The lowest temp I have ever seen since coming here in 1969 was -8C, and indeed EGKK is -8C right now
The service instruction from both Lycoming and Continental states that preheating is required by minus seven to minus 12 degrees. It doesn't say it shouldn't be used at higher temperatures, or that it's not a good idea. Anytime the temperatures are freezing or lower, preheating is a very good idea.
However, as you noted temperatures presently are minus eight, then you're a degree colder than the required preheating temperature for Continental powerplants.
No idea where this story originates but it could be with the pre-1960s crankshafts which had what I vaguely recall were calles slush tubes and those had a history of getting bunged up anyway;
Actually, modern, current crankshafts applied to constant speed propellers use sludge tubes, and they do require cleaning at overhaul, as they do contain sludge. Hence, the name.
Somebody I know up north is apparently seeing -19C but there is no chance of anybody driving around up there anyway. The car door doesn't even open, I hear.
If I tried to open the door on the plane in these temps, there is a good chance the door seal will be stuck to the other half and will rip off.
This really makes you wonder how the rest of the world manages to get along in cold weather, doesn't it? Operations in much lower temperatures than that are common in some places...without ripping off door seals...and yes, preheating is standard fare. You might even be shocked to know that in many locales, preheating automobiles is common.
If one was parking outdoors and had an engine heater out there, unattended, the engine heater would get stolen pretty quick (at all but the H24-secure airports, and most of the latter do not allow GA parking anyway).
The intelligent person would put the preheater in their car, or leave a lockbox in the tie-down spot, like many owners do. Perhaps that's too much progressive thinking for the UK, though.