You should use 20w 50 motor oil in it. "SL" is just the API oil classification from about 2001 - 2004 or so. Today's oil is "SN" rated.
If the Rotella is 20w50 then use it. If it ain't, then don't use it.
But, hey, it's your unit, put what ya want in it.
" But let me tell you something ,folks: You can't fix stupid!
There's not a pill you can take, there's not a class you can go to. Stupid is forever!"
" Pay no attention to that man behind the curtain!