May have had something to do with the quality of the second oil in the work car?
I know that some of the oils that I have tried in the Magna (265000kms) have had very different results as to "usage". Most of these were the same weight as well.
eg, Nulon "synthetic" 10w40 (dropped after 6000kms due to ticking) - 5L went in and only 3 1/4L came out. Valvoline Synpower "synthetic" 5w40 (full 10,000kms) - 5L went in and ~4.8L came out. Mobil 1 (5w50), Amsoil (10w40) and Motul 8100 (5w40) - 5L in and 5L out after 10,000kms.
Do you mean that the sludge built up when using the first type of oil and then the detergents from the second type cleaned it all out which caused the engine to use more oil?
I had my Stag for quite a while and tried a number of different oils (Amsoil, Shell, Mobil, Motul, Redline, Valvoline, RP, Penrite, Neo)... all different quality and very different detergents/additives. At the 220,000km service, all 5L of Sougi came out. Take from that what you will