Quick question for the experts

My RX300 has been running on 75W/90 Synthetic Gear oil on the rear differential and for the front transfer case ( manufacturer recommend 80W/90 regular gear oil but everyone recommend the 75W/90 Synthetic) if I only change the transfer case front gear oil, can I go back to the 80W/90 regular gear oil or must I use 75W/90 synthetic only as well for the front transfer case like I have been using currently if I don’t change the rear differential as it is not time to change the rear differential? Thoughts?

in short , do they both have to match in weight and synthetic ?

thanks everyone and hope I make sense

 

They do not have to match. I have been down that road when I used to live south of the border 20 years ago and experimented with this several times.