Oracle Database Discussions

Hi,
Practically , is there any difference between the two versions of row locking methods...????
In my case i need to lock only the rows which are going to be fetched by a cursor...but because the select stmt uses the sum aggregate function... I cannot use the format:

cursor c1 
 is
  select x , y ,sum(a)
    from d
   group by x,y
    for update;

So , the only alternative i have (i think) is the "LOCK TABLE d IN ROW SHARE MODE"
Is the above correct...so as to get the same row locks as the "select .... for update"????

Note: I use Db10g v.2
Thanks ,
Sim