I bought my truck new. It has 22K miles on it. I have changed the oil several times but was lazy and did not think it was necessary to check the rear diff. oil level. Finally, I checked it and it was to far down to touch with my pinky. I went to the stealer and he gave a little over 1/2 quart of the GM syn. oil. I put it in and can just barely touch the oil with my pinky. The oil is gray colored. I put some on a white back ground and used a light to see if it had reflective particles in it. Did not see any. I know I screwed up to assume that the GM factory filled it. I see no evidence of any leaks. WHAT SHOULD I DO NOW?????