I would just go to Walmart or one of the box stores and get motor oil made for power lawn equipment. I would think a straight 30 weight would be better than a multiviscosity oil. Just a guess there since all mine are 2 stroke mowers.

5W30 has the best tempertaure range. Synthetic oil is absolutely a foolish thing. Your engine will not last any longer with synthetic oil. It will probably last less because manufacturers will have you believe you can go 4-6 times longer than non-synthetic between oil changes. Take some sand in your palm and rub you hands together as hard and fast as you can. That's your engine running on old oil.

I suppose Briggs & Stratton is foolish with their oil recommendation for their engines. They recommend, #1, 5W30 SYNTHETIC oil for all temperatures between -20 and +120. I have used M1 in my lawnmowers for many years. My current mower, which is equipped with the Honda GCV 160 engine, was purchased in 2000 and presently is approaching 500 hours of use. I have an hour meter on it, (they usually require replacing about every 5 years, by the way) and it shows that it is used about 60 hours a year. In this 60 hour OCI, it has never been down more than a sixteenth of an inch on the dipstick.

I used Mobil One for many years in my personal autos, from the time it first came out back in the 1970's. It was said to be good for 1 year or 25,000 miles, and I took them at their word. My usual OCI was 18,000 to 20,000 miles, and I never had a lubrication problem, always a clean, quiet engine at as much as 140,000 miles, which was about as long (7 years) as I ever kept a car.

As far as old (long OCI oil) having sand in it, that is why oil and air filters are there in autos, and air filters are there in small equipment engines.

This enigine has an incredible dual air filter system better than any ofmy cars. I do realizethat you kick up more dust with a mower than a car as well. I have put Mobil 1 10W30 in it and it runs fine. Difficult to read the dipstick as the oil is so clear on it. Also this engine has an oil filter to remove any partcles that do get by. It is much smoother than any of my previous mowers and can mow really fast even in deep grass. No bogging just goes right through it and climbs hills great. I am very impressed so far. It was hard to get used to the very short pull on the recoiler. It only pulls about 6-10 inches which is less than half of my Briggs motor. Seems pretty good so far though. Oh and it has a drain plug on the side by the oil filter making changing a breeze. one thing I did notice that was strange ws the manual claimes it only takes a half quart of oil or 500 mL. Not much oil for having a filter on it as well.

If it were me, I would put in the Mobil 1 10W30 and not worry about it. Engines that have been run on mineral oils all their life (like straight 30wt oil), usually have a build up of varnish and crud around the seals. Mobil 1 tends to clean up that build up over time.

By the way, they make a Mobil 1 10W30 High Mileage version. It is a even smarter bet to put in a used mower engine. It is more "stout", and a bit thicker than the standard Mobil 1 10W30. It is less likely to be "consumed" after hard usage. Also has seal conditioners in it.

I use it in a 15kw, Generac, air cooled, natural gas fueled, standby Generator. 50 hours of non stop operation in 90F temperatures is a lot harder than running a mower a few hours at a time.

See my post above. That Mobil 1 10W30 High Mileage oil is what I am now using in my auto and all of my outdoor power equipment. It is still an SL oil, not an SM that has been decontented to extend the life of catalytic converters in autos. The special synthetic oil sold by Briggs & Stratton is still an SL oil, as is the Amsoil 10W30/30 oil de3signed for small aircooled engines.

Use what the owner's manual recommends! Best advice anyone can give! If the mower is new, use synthetic and change it half as often as you normally would. Full syn oil resists oxidation and high temps much better, therefore, hold up longer. I worked in an oil blending lab for a while, and there's some poor advice being given here and there on this topic. During warm weather there will be little difference in 5w30, 10w30, and straight 30w. If, however you use your lawn tractor to move leaves, firewood, etc. in winter temps, be sure to use a 5w30. The numbers represent the cold temp performance and the running viscosity (at 100 degrees C). So a 5w30 starts up like a 5 weight oil and gets no thinner than a 30 weight at running temps. Since most engine damage occurs at start up before the oil gets distributed, the 5w30 helps mitigate damage during the delay. Semi-syn make very little sense to anyone except marketing people who want your extra money just to put 10% syn in a blend. High mileage oils contain a "seal swell" agent to condition rubber o-rings, e.g. distriutor o-ring, preventing some leakage when they wear. Not needed on newer stuff and a good idea on older equipment. I really do not know if there are rubber seals in a mower?? Thanks, my 2 cents.