OK.. everyone is going to hate me and disagree with what I'm about to say, but the following info is a fact based on my experience with driving over 200,000 miles on a H55F WITHOUT the plastic oiler cup.
It's not required.
Desired? Yes
Recommended? Yes
A good idea? Yes
A better oiling provision? Yes
An example of Toyota over-engineering? Yes
Required? No
When I purchased my H55F in 1989, it had to be imported directly from Japan. No one was selling it here in the USA. The Toyota parts person who ordered it for me did not know about the transfer oiler cup, nor did I. It never got ordered or installed when I put the thing back together (Doh!). The extra hole in the transfer case I did figure out to plug up though.
Fast forward 26 years later and over 200,000 miles.... I saw a picture of that little plastic oiler thingy on the web and thought WTF???
I just had my tranny out to replace the transfer seal and installed the new little plastic cup. Damn right I did.. and I inspected the 5th gear.
It has never popped out of gear since day one,
Has always been quiet and is quiet to this day,
Always has shifted perfectly into 5th
And upon inspection a few months ago................ it showed
no signs of wear at all. None.
My conclusion based on this 26 year/200,000 inadvertent "test" due to a mistake is that the little plastic oiler cup is not required, but definitely a good thing to have.
I would recommend installing it for sure. But if it's not there, no harm, no foul. Can't argue with 200,000 miles.
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5th gear close up after 26yrs/200,000 miles (not all in 5th gear obviously). It looks almost new.
5th synchro engagement teeth