If the guides are severely worn new valve seals may not improve oil consumption or only improve it marginally. My Cosworth Vega uses the same plug (geometry and heat range) as the M103 (except a resistor version and the NGK TR5 is my choice), and it got down to a quart every 200 or less before I pulled the head and replaced the guides and seals, but even at that rate of oil consumption it did not foul the plugs and ran great.

Duke