Full-grain leather is low-maintenance, not no-maintenance. A few small habits keep it supple and let the patina develop cleanly. None of this takes more than a few minutes.
Everyday
Let it breathe. Hang the jacket on a broad hanger, not a wire one, so the shoulders keep their line. Keep it out of prolonged direct sun and away from radiators — heat dries leather and fades colour. If it gets caught in rain, don't panic: let it dry naturally, flat or hanging, away from any heat source. Never force-dry it.
Now and then
Wipe dust off with a soft, dry cloth. A few times a year, or when the leather starts to feel dry, apply a small amount of a conditioner made for fine leather — sparingly, with a soft cloth, testing an unseen spot first. Sheep leather is thin and supple, so less product is always more.
Marks and storage
Blot spills; don't rub them in. Leave scuffs alone — on full-grain, most settle into the patina rather than ruin it. For long storage, use a breathable garment bag, never plastic, which traps moisture and can encourage mildew.
That's the whole ritual. Do this and a full-grain sheep leather jacket will look better in five years than it did in its first week.