Traditionally it was considered good to lead the highest of three when leading partner's suit.
The first caveat however was not to do so when you hold the unsupported K or Q - as it would be a shame to give declarer two tricks if they hold the AJx.
What is your opening lead from J63 if this is partner's suit?
"Even from J63 it is best to open the lowest. The suit may be distributed like this:
H J63 H K9852
The lead of the jack will give South an additional trick...
Occasionally, the bidding may indicate that dummy holds an honour in the suit. Then lead the jack or queen, because you want to play through dummy. The situation may come about when the bidding has dummy bidding NT at some point."
This point and some of the text comes from Victor Mollo and Nico Gardner Card Play Technique (2013, Master Point Press, pp130-1). Copies are held in our library.