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It's an old bike; I'm afraid if I go any less that 25:1, it'll seize. Unless I use a good oil?

I've just got a Yamaha TY80 for my son and was wondering what the oil / petrol ratio was? The oil / petrol is mixed in the tank and the closest guess I have is 20:1 which is what my FS1-E was, back in the day> Also, the bike would have tun on leaded fuel. Do I need to get a fuel supplement for unleaded? I haven't ridden trials since the 80s (I used to have a Bultaco and a Montessa) and so am a bit out of touch. Thanks.