Castroil
Yes you can use 2 cycle oil to mix with the gas for a mini pocket bike.
When you change the oil put in 0.53 qt. (0.5) L
check your oil level it sounds like it is over full
To way to fix that ... eiter change the oil or your ckoke may be broken I have the same bike and that was the way i fixed it.
The precise type of oil that a Yamaha TTR 90 dirt bike should use can depend on the year the bike had been manufactured ad should be listed in the manual for the bike. For a 2002 Yamaha TTR 90 bike, the recommended oil type is 10W-30 (when 10 degrees Fahrenheit or above) and 20W-40 (when 40 degrees Fahrenheit or above).
MINI specs synthetic oil for all models assuming you are talking about the moder MINI and not the older Classic Mini. Using a normal dino oil will void the warranty and not offer the same level of protection a synthetic oil provides.
grass and oil
Depends on the manufacturer as to what type and weight oil to use.
Consult your owner's manual.
I OWN THIS BIKE THERE IS A SMALL NUT LOCATED ON THE BOTOM OF THE ENGINE DOWNLOAD THE MANUAL FROM www.bajamotorsports.com TO SEE WHERE EXACLY IT IS
SAE 140 1 qt