Our oak sills are rutted from years of wet and frost. I feel I should sand them down and treat in some way, but mindful of the mantra that old houses must breathe and not trap moisture in. I read that boiled linseed oil is the choice for most wood, but not oak. The previous owner said they are over a hundred years old already and will cope. Is there a correct process for oak sills under leaded windows?
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LadyArowana
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Can't help with a good answer, but bumping this up for a new member. Hello and welcome 
88v8
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In our previous house, I treated external oak with teak oil, which is basically a rubbish substitute for linseed.
I would suggest a 50/50 mix of boiled linseed and turpentine (not Turps Subs). The turps will help the linseed penetrate. Both are available on't'net.
I *probably wouldn't rub down, the texture is part of the charm. Too easy, little by little, to erase the charm that attracted you to the house in the first place.
*Probably, because it's important to maintain the weathering (fall) on the sill profile. If water has rotted a depression in the sill such that it's become flat or even dished, I might get out a chisel and see if I can make it sloped again so that the water runs off. Otherwise, I'd leave it alone.
Edit... another thought... looking just now at the C18 door I put in the garage and treated with linseed/turps, the oil significantly darkened it. Now, I'm not saying that's bad, just that it happened. So, if you do the sills, you might want to think twice about straying onto the frames, otherwise you could find yourself having to treat all the frames on that front or even the whole house, to avoid the colour difference.
Quite possibly, the wood has never been treated, so changing its colour is not something that should be done lightly. So to speak.
And, yes, welcome.
Ivor
I would suggest a 50/50 mix of boiled linseed and turpentine (not Turps Subs). The turps will help the linseed penetrate. Both are available on't'net.
I *probably wouldn't rub down, the texture is part of the charm. Too easy, little by little, to erase the charm that attracted you to the house in the first place.
*Probably, because it's important to maintain the weathering (fall) on the sill profile. If water has rotted a depression in the sill such that it's become flat or even dished, I might get out a chisel and see if I can make it sloped again so that the water runs off. Otherwise, I'd leave it alone.
Edit... another thought... looking just now at the C18 door I put in the garage and treated with linseed/turps, the oil significantly darkened it. Now, I'm not saying that's bad, just that it happened. So, if you do the sills, you might want to think twice about straying onto the frames, otherwise you could find yourself having to treat all the frames on that front or even the whole house, to avoid the colour difference.
Quite possibly, the wood has never been treated, so changing its colour is not something that should be done lightly. So to speak.
And, yes, welcome.
Ivor
Keith Bowman
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My experience/understanding is that boiled linseed oil sits on top of the wood and acts like a varnish and raw linseed oil soaks in. Adding turps to boiled linseed oil to make it soak in seems a bit of a roundabout way of doing things. Or am I wrong, as is usually the case. And, I think teak oil is simply raw linseed oil with turps (or more likely turps substitute or white spirit) added. I expect the quality depends on how much they dilute the linseed oil.
88v8
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This is another of those subjects where it become confusing the more one reads.
There's a Wicki article, which begins by stating that the oil is hydrophobic, a quality which in my vague fashion I would have ascribed to all oils, but later avers that the finish is easily penetrated by water.
It also mentions that boiled oil is less inclined to yellow, although I would reckon that more an issue indoors where it is not exposed to bleaching.
I see that boiled can be used to adhere gold leaf. Perhaps that is the answer; oil the sills and cover them in gold leaf.
Might look a bit 'Kev', I suppose.
Ivor
There's a Wicki article, which begins by stating that the oil is hydrophobic, a quality which in my vague fashion I would have ascribed to all oils, but later avers that the finish is easily penetrated by water.
It also mentions that boiled oil is less inclined to yellow, although I would reckon that more an issue indoors where it is not exposed to bleaching.
I see that boiled can be used to adhere gold leaf. Perhaps that is the answer; oil the sills and cover them in gold leaf.
Might look a bit 'Kev', I suppose.
Ivor