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Re: Confused about which oil weight to use
The main difference is in the cold start... which in most pleasure craft will not be all that cold.
I've used Rotella T 15w40 in EVERY engine I've owned for the past 35 years. I get 250,000 miles out of every vehicle. But 97% of my miles are low rpm, seldom over 3000 and never over 3500. Even on the lawn mower I do not run wide open... 80% is my max. Once warmed up its in the 15 to 20 wt zone. 40 is at cold temps at whatever the standard temp is. They are all almost grease at below 0, but I highly doubt you'll be sailing at 20 below. Start and let it warm up before working as is proper for any engine and I expect you'd be fine. This doesn't work for some of the modern high rpm automotive engines though. They are a different animal.
Most of the marine sail applications on general use are likely low rpm high load... exactly what the Rotella 15w40 was designed for. It will keep a Mack 350 semi engine going at 100% for a million miles before a rebuild. Been there did that.
Now there is another potential issue though... that of detergent vs non-detergent oils. On many old engines pre 1950 or so that ran non-detergent is the shellac and gum that is actually doing the ring sealing. Switching to a detergent oil and they can start burning alot more oil. Its doubtful your in this category though.
1980 Seafarer Swiftsure 30
1978 Bayliner Buccaneer 270 (now sold and being restored in FL)
1962 SeaMac 14' Plywood Runabout, mahogany decked, with 1959 Evinrude 35 Big Twin (owned since age 17, I'm now 60)
Last edited by dem45133; 07-31-2013 at 12:13 PM.