VZ Commodore oil consumption

We have a 2006 VZ Commodore SV6 that has done 115,000 km and is using 3.5 litres of oil between the 15,000 km services. The dealer who has serviced it since new claimed this was within GM's guidelines, which was confirmed when we contacted GM. I inquired what was an acceptable level of oil consumption and was advised it was analysed on a case-by-case basis. It may not have any relevance, but the engine had to be pulled apart at approx 50,000 km as it had an engine knock. It seems oil was starting to be consumed after this and the dealer was notified. Have we got a lemon, or is this normal and acceptable usage?