I am changing the lower unit grease on my Johnson 9.9. While at the store I noticed two types of grease. One was just regular SAE80W-90 GL-5 and the other was "Marine Grade" SAE80W-90 GL-5. The only difference I can find is price. Marine Grade is $3 for 8oz and the regular is $3 for 1qt. Is there a difference? Must I use the "Marine Grade"? Thanks!