I run a 99 Opti, and for the longest time I used premium plus. but switched to Opti oil, But I added the one to the other. I looked at the oil today and it looks very dark. Is this normal, not sure if I should take it out and put new stuff in , but its very expensive ! is there a way to check the oil ? I put some in a jar and I'm going to take to the mechanic tomorrow. thanks