It's normal. Gear oil can be whipped up by the gears pretty fast. There are some videos on youtube of people who have put on clear differential cases. You can see it directly. The oil starts out fine, and within a minute it looks like a frothy milkshake. This is normal. There are anti-foam additives in GL-4/5 to keep it from getting out of control.
Cost of Hobby=2L
(Where L=Cost of Lubricant)
95 Lexus SC300 5-spd@200K, 2000 GS300@125K both VML Full Syn+Purolator PSL20195(soon to be Wix)