changing oil and weird tranny noise

[SVXMAN2001]

I was just wondering what the best brand of oil to use is, and what weight. Also I have a weird problem with my tranny, I hardly ever do but when I floor the car after I come to a stop and get going again, I get a horrible grounding noise, I was thinking it might be low on tranny fluid but just thought I would ask here first, it's a rebuilt tranny with about 7k on it

thanks in advance

chris