In my experience dealing with automotve parts data in an inventory control system and online I have realized the importance of relevant alternate and recommended items.  These are very useful to drive your online vistor to add more to their cart at the time of sale.

A good example of this can be seen here where this Auto PlusXP user has linked an alternate part number and recommended items that are relevant to the main item.

This gives the online user options at the main part number level to add associated items to their cart that they would in most cases purchase in the near future from your site or someone elses.

The more relevant content the better to give your online customer more related options without having to search all over your site on their own to figure it out.