Just a thought. Sorry. I don't fallow the net as much anymore. This may have been covered. But. You can only change to just synthetic. Is at a total trans rebuild, and converter flush/replacement. When you change fluid, and filter. You're only able to drain around half the old fluid out. You'll never get to 100% of either. While converting.
I rebuilt my 425 trans in 2004 myself. Then Installed it with an Allison converter. That trans is now in our second 78 Royale. With around 100,000 miles on it combined. No synthetic ever used. My minimum GVW is at 17,000 lbs. Max is at 24,000 lbs. Change fluid ever 12,000 miles. When in the mountains. NEVER use more than half to 5/8 throttle. Shift to a lower gear. The keep RPMs in the 4400 to 4800 RPM range. Seldom above 5000. 3:70. 403.
Bob Dunahugh
78 Toyale since 2003
4 real COPO Yenkos
I rebuilt my 425 trans in 2004 myself. Then Installed it with an Allison converter. That trans is now in our second 78 Royale. With around 100,000 miles on it combined. No synthetic ever used. My minimum GVW is at 17,000 lbs. Max is at 24,000 lbs. Change fluid ever 12,000 miles. When in the mountains. NEVER use more than half to 5/8 throttle. Shift to a lower gear. The keep RPMs in the 4400 to 4800 RPM range. Seldom above 5000. 3:70. 403.
Bob Dunahugh
78 Toyale since 2003
4 real COPO Yenkos
