If using a lot, I would just save up for a bit and do the lot, I have learned the hardway, as mine was burning oil from the top and bottom end, although it was running fine, so I did the head thinking it was guides, only to find only a small difference, have now ripped engine apart and am going for rebore and rings, I did find though that changing from synthetic to a valvoline 20/50 racing mineral oil reduced my consumption a lot, and its only £20 for 5 litres, so worked out a little cheaper.