A mechanic friend of mine (Nissan) says that tranny fruid (ATF) is nothing more than 5W-20 without the dispersants. If this is true, why is it so much more expensive than motor oil? It makes sense though. The tranny fluid cools and lubricates. The engine oil cools, lubes and traps blowby gas contaminants.

 

I believe transmission fluid is more of a hydraulic type of fluid meant to be used under pressure to open and close valves.

 

Transmission fluid is hydraulic fluid with additives. Hydraulic fluid is very similar to light weight engine oil. Transmission fluid (in bulk lots) does not cost partiularly more than engine oil. Name brand, high quality engine oil costs about $1.60 a quart. Similar transmission fluid costs less than $2.50 a quart. The difference is probably more attributable to quantities sold than anything else.