I'n not sure about that. I have about $65 into an oil change on my Merc 150 4S every 100 hours, or end of season even if I don't have 100 hrs on it. Do the modern 2-Strokes use that much oil in 100 hrs of general operation?
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One thing I will add to this topic is this: A few years ago I had a Merc 150 4S blow a walnut sized hole in the block with just a few weeks left on factory warranty. Mercury took fuel and oil samples to make sure it was going to be covered under warranty. While I do NOT know what they were looking for in their samples, I do know I was running Merc 4S motor oil, and they never blinked an eye at replacing a roughly $9600 powerhead through my local dealer.
Now, if'n I had been running some other brand of motor oil, who knows what the warranty experience would have been like. I do know I was sitting on pins and needles awaiting Merc's determination on if they were going to honor their warranty.
In a nutshell, if the motor is under warranty, do you really want to risk voiding the warranty by running something that isn't technically their brand, even if you have had great experiences with a particular brand in the past? That bill for a new powerhead would have sucked on a brand new to me rig, I think I had put around 30-40 hrs on it in those short few weeks I had it before the motor went kapow.