Source: Ginny & Jeff Schuett para 
Opening Leads versus slam contracts
Opening leads in general are tough and slams offer even more difficulty in making a good lead, but it is important to listen to the bidding and make a few judgments to start.
The issues of leading against slams are fundamentally different from leading against games or part scores. If you are on lead against a small slam and hold five to eight high card points (HCP), it is less likely that partner has much, or anything. So under-leading an honor that you might do at lower levels makes less sense, because you are far less likely to “hit” partner’s honor, if any. But if an aggressive lead is needed, it is still worth the risk.
In general, the critical part of choosing a good lead is deciding whether to make a passive or aggressive opening lead. Leading an unsupported ace or under-leading a king or queen is aggressive. Leading a trump from two or three small is passive.
Listen to the bidding for important clues – unbid suit(s) are often the best alternave(s).
Aggressive Opening Leads
Try to determine from the bidding if one of the players has a long side suit that might be used as a source of discards. If so, then an aggressive lead is called for to develop possible trick(s) before the long suit provides discards. For example:
In both of these sequences, the dummy is likely to have a long diamond suit as a possible source of discards, so an aggressive lead of one of the unbid suits is called for. From a hand like: 
T9852 
A5 
87 
QT72 a lead of T is safe, but not likely to lead to a trick, but 2 needs less from partner to lead to a trick.
If your side bids a suit and one of the opponents jumps to slam without using Blackwood, suspect a void in your suit, and try leading a different suit. Often Blackwood would be used if there is no void.
If you might have a trump trick, or it looks like partner might have one, lead a side suit ace. Especially at matchpoints, often just getting your ace might lead to a pretty good score.
Passive Opening Leads
In general, leading against 6NT or any grand slam should suggest you lead very passively. Leading fourth best from any broken holding makes little sense, and will very often mean giving up an important trick.
Do both opponents seem to have relavely balanced hands? Bidding like:
shows they are both likely balanced and also they likely have at least 32+ HCP combined. If they are both balanced, it is less urgent to make an aggressive lead, or lay down an ace.
If one hand has indicated great strength (like opening 2) and the other great weakness (like responding 2 to show a bust), it is usually right to defend passively. A very weak hand rarely provides lots of discards.
If the Slam is Doubled
- If partner hasn’t bid, lead dummy’s first bid suit or naturally shown suit – the Lightner Slam double.
- If partner has bid, usually do NOT lead partner’s suit. Often this is a warning to lead a different suit, implying a side suit void, or honors in dummy’s first bid suit.