Whwn i bought my truck it has 12,xxx mi on it , The guy before me used mobil1 for the first 3 changes. I noticed that the dealer put back in regular oil after running full syn for it first 12,000 miles. IS that bad for engine? i just bought mobil one today and a new filter to switch back. of should i just run the next 3,000 mi then change back to mobil 1?