Probably time for some new glow plugs. Ford had a problem with their 6.0l engine and fuel injectors. The oil would sit in the injectors and make it difficult for them to start once the oil thickened up, they had a recall that would spool the injectors when you shut the engine off to get all the oil out and it worked real good to fix the problem, maybe Chevy has something similar. Also, if Chevy uses oil pressure to fire the injectors like Fords, low oil pressure could also cause hard starts.
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