I can recommend Shop4parts, bought quite a bit of stuff from them and no problems at all - good quality stuff but sometimes brands I'd not heard of before, I think because they source some stuff from Italy direct. "Original Birth"? I was very suspicious but all good! Very helpful people on the phone too if you feel like giving them a ring. Other than that I buy from local trade factors - the source of most parts for local garages around me. I find they are pretty much similar to online prices or only slightly more but there's the advantage that, if a part is wrongly supplied, you can just nip back with it and in very short order you're back working on the vehicle again.
Tipping a tin of additive in the sump and hoping it's going to work miracles? Probably unlikely. I've found products which claim to reduce oil leaks/consumption tend to work on either thickening the oil with lots of polymer and/or they contain an ingredient which softens the seals - seals harden with age which is often why they will be leaking. I don't like the idea of thickening up the oil because that can lead to oil starvation of various parts inside the engine and I don't think much of "softening" the seals because it's likely to be a very short lived "cure". I suppose, if you're unscrupulous and just trying to get the car sold, you could chuck a tin or two in, but if you're looking for a long term sort then you've got to address the root cause.
