well it'a a domestic line trimmer, how much fuel can you possibly run through it?
1. Could the price difference matter?
2. If you DOUBLE the oil in the fuel, you double the specific price of that cheapo oil use. Is it still "cheap"?
3. If you use too much oil, you could suffer bad smell and coked plugs.
But I agree, in this case you could ignore the "synthetic" remark, look at oil ratings instead and if you find an oil with modern ratings and with recommendation to run it at 50:1, or 2% as we say here, buy it and use it.
I would look at house brands, and maker's brands, of chain saw oil at big box stores. Say a FC rated oil will work veyr, very well at 2% too, and will be reasonably cheap.