I have done a lot of research on the L18 and had a few conversations with Indmar engineers along with marine engine builders. Indmar changed their oil requirement to straight 40 wt. because folks weren't checking their oil levels during the summer and the L18 will use some oil and the switch was to help prevent the slight oil usage. I feel, and so do some of the engineers at Indmar that 15-40 is better and just check your oil more often. The GM 8.1 has a 8 quart oil pan but Indmar states 10 quarts once again because folks fail to check their oil level during the summer and the level drops and Indmar wanted the extra oil for safety.
After all that babble - just run 8-10 quarts of 15w-40 and check your oil level often.
2008 X-45 Pro Tour Red and White