Small Engines (Lawn Mowers, etc.): Black smoke (fuel would burn), viscous oil, acceptable substitutes

Question
Hi Ben,
We bought second hand lawn mower Mastercraft 6.25 HP.It was working fine until I added oil. Mower didn't have manual, oil was low and I didn't know what type to buy in store. I found 2 types: 2 cycles and 4.  I was not sure which one is suitable, anyway, I bought the one for 4 cycles (sticker on mower says only SAE30)
Now black burned oil comes from muffler and engine works very bad, choking.  If this is the right oil, what can be the problem?
Also, if oil needs to be changed, how much oil does should be put in (indication stick is broken)

Thank you in advance

Lior

Answer
lior
Use the recommended oil for your engine. SAE30 is usually specified by the engine manufacturer but they will usually state that 10W-30 or 10W-40 (SG rating) are acceptable substitutes. These 'multi-weight' oils are widely available and inexpensive. Typical small engine oil capacity is just over 1/2 quart. Start low and incrementally add oil until just below full on the dipstick or at the top of the oil fill hole if there is no dipstick. DO NOT overfill. Give it time for the relatively viscous oil to find its way into the crankcase. Tipping the mower back and forth a bit will help this process.