Re: Why a single viscosity oil vs multi?
What the multi viscosity does (or is supposed to do) is to perform over a potentially wider range of operational temperatures. In cold weather, the oil will flow easier- that's the 10W aspect, and at high temperatures, act more like 40W. If your specs call for straight 30W, you'll be fine.
BUT you're really NOT telling the whole story. The Non-synthetic multigrade oils such as 10W-40 is REALLY a 10w oil.........then they add VI "improvers" to increase the viscosity when the oil gets HOT.
(works great for automotive engines that sometimes start at freezing temps........(when was the last time you operated your boat in freezing or below freezing temps?)
As the VI improvers "break down"(I.E. the oil doesn't "stay in grade" for the given temp), they become unable to maintain the the higher viscosity at the higher temps where just about ALL marine engines operate anyway.........AND, which is really why a single grade oil is probably "best" for them.................
(excluding synthetics which do not use VI improvers.....and which might explain why Volvo and Merc use certain synthetics or semi synthetics)
You could also use 0W-40 or 0W-30 (synthetic) and basically get the same performance as a straight weight. I don't think you're going to get much of a benefit in a marine engine though....... You STILL need to change it every year (so it sits all winter with fresh clean oil inside)
Synthetics don't thicken at extremely low temps (take a look at the kinematic viscosity test results for Mobil 1, Amsoil and other full synthetic oils.....0W-30 for example has the SAME viscosity at 100*C as straight weight "reg" SAE30 (or it wouldn't be "SAE30") but doesn't thicken at extremely LOW temps.
Doesn't 10w-40 protect better than just 30w?
Not really.
Multigrade oils are made for a couple of reasons.
1. Winter use. If you use a "multi" in sub freezing weather, you get immediate "FLOW" of oil to where it needs to go......... Using a straight-weight you might get immediate pressure but the oil is so THICK that it DOES NOT flow right away to where it needs to be down-line of the pump like the furthest away rod or main bearing..............
(some radial aircraft engines back in the "OLD-days" used a fuel-oil dilution system where the pilot would pull a knob in the cockpit and dump AVGAS into the 50W or 60wt (AV grade-100/120 ) oil a few minutes before engine shut down. This would dilute the oil enough to make it "runny" for the morning start up.
After warmup, the gasoline would simply evaporate out of the oil. (A sort of "poor-mans" multi-grade oil!!)
It worked great except for the lead and other "stuff" that was added to the oil.......(and never mind the fire hazard ADDED to the oil
........radial engines never leak do they?
.......LIKE A SIEVE
!!!!)
2. The main reason is fleet fuel economy for cars....
CAFE ...... (political) The lower weight oils allow for slightly higher fuel economy. It's pretty much accepted that 5W20 for a car produces the best fuel economy.......10W30 (or straight 30, 0W30) will "protect" against wear better......The auto manufacturers want increased CAFE. It's a trade off but most people don't keep their cars long enough to wear out the engine.....and manufacturers don't really care much about the second owner!!
Bottom line..........If you don't want to use the oil that Mercury or Volvo Penta suggests, you can probably use whatever you want and "never have a problem"..............
Marine engines usually fail from freeze cracking (block) and/or LACK of maintenance not what type of oil is in them...
ymmv.........
Rick
