Oil & UK issues
As has been said above there is no problem with mixing Multi grade and other aviation piston engine oils, however I would not bother if the engine is more that 50% through it's life as the advantages of the Muilt-grade are unlikely to be of any benifit for the increased cost and the multi grades do have a habit of "cleaning" the engine so you have to inspect the filters more regularly over the first 100 hours or so when changing to multi grade.
The lead thing is a red herring. lead build up in the oil ways was as a result of using a pure synthetic oil in engines running leaded fuel, synthetic oils can't hold in suspension the lead that has blown by the piston rings and so the lead is centrifuged out of the oil in the rotating parts of the engine hence the lead build up in the sludge traps and eventualy the crankshaft oilways. The major oil company that was marketing FULLY synthetic oil should have known better!
Semi-synthetics are OK as the mineral part of the oil can hold the lead in suspension untill the oil change at 50 hours, I would not run a semi-synthetic oil past 50 hours as the ability of the mineral part of the oil to hold the lead in suspension is limited.
Mr Guppy seems to be unable to understand that the climate in the UK at the moment is unusual in and so it is highly understandable that we are about at prepaired for it at Hawaii would be for snow. the other thing that he forgets is that the VOLUME of the water content of the presipitation and could air is much higher than the "dry cold" of the mid continental USA and brings different problems.
We also have a bunch of reguators who are (in EASA) just pen pushers and so don't understand the first thing about aviation and so the cost of fitting a pre heat system to an aircraft is not viable for the few days when you might need it. That is always provided the runway has been cleared of snow!
We may have to review these things over the next few years in view of climate change, IO540 is on the south coast and had -8C last night I am only about 70NM inland from him (EGTB) and had -14C last night so I think things are changing.