Re: 40hp 4 stroke...how much engine oil??
If you have a dipstick, most 4 cycle engine mfgrs. specify the correct level by following the reading on that regardless of "the oil capacity". I like to introduce new oil into the new filter prior to installation, like half full or so, fill the engines to the F mark on the dipstick, crank and run for a minute or two, shut off, wait 5 minutes or so and then top them off at the F mark.
Since marine engines operate at elevated rpm's as compared to other engines used in cars, trucks, lawnmowers, etc., one would imagine that an oil with adequate surface tension and additives would be necessary. Merc markets 4 cycle marine oils for that purpose which sometimes can be purchased at Wallyworld or surely at your marine dealer or www, possibly here.
I would assume that a multiviscosity oil would be in order, given the temperature extremes that a lot of OB engines encounter, like 5 or 10W-30.
If you can't find that then I PERSONALLY wouldn't have a problem with Mobil 1 5w-30 if you live in the northern regions or 10W-30 in more tropical regions. OEM mfgrs are finally adding multiviscosities and synthetic oils to their recommended oil charts....about time.
HTH
Mark
