Seals due tend to wear out, some sooner than later and heat plays a big roll in the time frame. Also if any debris gets caught up behind the propeller/on the prop shaft for example, it can damage the seal. Another area of concern is the input shafts U-joint shaft "yoke" (item # 26)
http://www.mercruiserparts.com/Show_...TER+COMPONENTS It's common for that area to show signs of wear damaging the seal in turn allowing oil into the U-joints bellows...sometimes oil is visible in the bilge area sometimes not.
In the early 90's I recall Volvo in general recommending seal replacement intervolves as a preventive maintenance but nothing in writing that I remember. Mercruiser could fall into the same category as a Volvo.
Some marine service shops will recommend new seals while others will say replace them when they leak.
If you do constant inspections of your boat before each trip and do preventive maintenance such as drive oil changes and pull the drive every year then you'd least likely need to change them yet but if your the type that don't do or inspect things more often than not (especially since your boat sits in the water most of the time) then yes it would be in your better interest to replace them beforehand.
Bravo 2's are typically designed for bigger heavier work loads which could equal more heat if the boat runs on plane a lot.
What model boat do you have? What speed do you travel?
