99 323i Manual tranny fluid and diff alternative to RL

Hi all. I have a quick question (I have used the search and did not find my answer, really I did). I currently have RL D4 ATF in my the manual tranny of my 99 323I and RL 75-90 in my diff, which has been working flawlessly for years (still shifts smooth after 100k since change). I am driving across country once again from VA to CA in less than a month and feel the need to change both fluids. I am on a budget and need a cheaper alternative to redline fluids? is there any (mobile, Castrol, etc.)? and how would the feel of shifting be with the alternative? Any help on this would be much appreciated.