If it is a North Star Cadillac engine the mechanic or machine shop is not to blame. I worked in Engineering for 7 years and can tell you the North Star engine is not a great design. The oil consumption and carbon issues and oil leaks are all design flaws. The piston rings are too far up the piston. The rings are low tension. The piston skirts are too short. The mating surfaces are not well machined and the engine uses too many small gaskets. Like all GM aluminum casting the grade of aluminum is poor and the porosity too high. I could go on all day like this. The North Star was one of the first engines to have insane oil consumption and carbon issues but later on not only did other GM engines have this issue but other OEM's started to have similar issues. The ones that did not had sludge issues. It all goes back to attempts to lower emissions and increase fuel economy. The latest problems are insane carbon issues and fuel dilution on direct injection engines and all kinds of EGR, blow by and carbon issues on turbocharged direct injection engines most notably on Fords Eco-Boost engines but not just Ford but all similar setup's from other companies. It always comes back to fuel economy and emissions in terms of problems with long-term durability vs power density. The Cadilac 4100 V8 was even worse. The N.Star was just bad. Smart money would have been to co-develop the Vortec engine and have their own DOHC set of heads for it and called it a day!