^ I stand corrected. I relied on my commercial parts account and this isn't the first time it's been wrong. Further research revealed Leeann is correct.
FYI - the A518 is, at it's core, an A727 with an overdrive and lockup torque converter added. The first three gears in both are essential identical, with most parts interchangeable. The A518 overdrive gear is in the tailcone, attached the the back of what is essentially an A727.
(Some late model A727 have lockup torque converter.)
The lockup torque converter in particular requires specific fluid, while the non-lockup A727 is less finicky.
The level of engineering and manufacturing of the A727 is demonstrable. It remained in production for decades, from it's introduction the early '60s until it was effectively replaced by the A518. It is also quite durable, as long as it's not abused. (I had to rebuild the one in my '66 Polara because it hadn't been maintained and was subjected to abusive driving.)
Unfortunately, the overdrive and lockup torque converter additions for the A518 didn't have the same level of engineering and manufacturing.
Early A518 torque converters suffered from faulty clutch material in the lockup torque converter. The material had a tendency to disintegrate and the particles collected in the transmission cooler in the radiator, plugging it. Because the torque converter continued operate like a non-lockup version, people weren't aware of the problem until the transmission failed due to excess heat.
Aftermarket torque converters were marketed to counter this problem. Likewise, Mopar upgraded the clutch material in the torque converter when it identified the problem.
Also, the overdrive in the A518 tailcone is not as durable as the A727/A518 parts ahead of it. Moderately aggressive driving (i.e., routine hard acceleration) can damage the overdrive.
It's possible to switch from an A727 to an A518 but I wouldn't recommend it, especially in a motorhome. There are alternatives to gain the overdrive and lockup advantages of the A518, using the original A727 as a starting point. (It's not cheap so the fuel savings may not offset the upfront cost.)
