1995 subaru legacy

recently purchased a 1995 legacy 4cyl all wheel drive with 90,000 miles. was told by a former Subaru employee that front diff, gearbox and back axle oils should be changed every 15,000 miles. is this just dealer hype? was planning on changing them because i have no maintenance records on this car but it is in excellent, “just taken to church on Sunday” condition. how many miles in between changes, if at all? thanks appreciate the knowing . Karen