Check the owner's manual, call the manufacturer or head to a oil lube shop and ask. Some makes and models, yes. It isn't a good idea to go with a heavier weight oil unless the manufacturer says it is acceptable. Sometimes the weight of the oil relates to the lowest weather temp the car will be exposed to, but sometimes an engine is not designed to handle another weight.
Can you use 10-30 weight instead of recommended 30 weight in a 26 horsepower lawn mower
Yes you can.
is it allright to use 20-50 engine oil if it states to use 30
Use exactly what the manufacture recommends.
No, it is not. Use exactly what the manufacture recommends. What good reason would you have for doing otherwise?
For normal climates, use 10W-30 oil.
You should use what your car recommends 5w-20 is thinner than 5w-30.
I would go with the 20 since it's thicker. 20 is NOT thicker then 30. Yes you can use a 30 weight oil with no problems. 20W oil may be recommended in order to achieve maximum fuel efficiency but, using a 5W30 oil will protect the engine better!
Use exactly the weight oil recommended by the manufacture and nothing else. Using anything else can cause engine wear.
10w 30 or 20
5W-20 or in hot climates - 10W-30
You should use synthetic oil 5 w 30 or 5 w 20 depending on the season