If you want to mill that 30" oak, realize that with a Grandberg mill you will lose about 4" of bar width due to the mill clamps on the bar, so a 30" bar will only give about 26" width max.
Also, a 50cc class saw will have a heck of a time pulling a chain long enough to get through a 30" oak in a mill. You will really run the risk of wrecking the saw, and it will take a long time for each cut even if the saw survives. Also, a 30"+ bar/chain combo will be a $100+ investment, not to mention a mill that big will be $200-300 more.
What you may want to consider is splitting the log in two pieces by cutting it halfway through down the center by making two passes (one on either side) with the tip embedded and using a shorter blade (20" maybe) and either a mini-mill or small log mill (neither of these attach at the tip, so you can sink the bar all the way in). Then, once its cut in two, you can mill out narrower boards in the other direction.
(This is using whats called a Logosol style mill and only cutting off the edge, but you can see what I mean by cutting on either side with two passes and embedded tip)
However, realize that on this initial cut this will still be
a lot of load on your saw, so you should take it easy, let it cool down every minute (just let it idle a minute or two and let its own fan cool it, don't just immediately turn off the saw), and therefore don't run out of gas while the engine is hot (milling can suck up the gas, so you may need to refuel mid-way depending on how long the log is). A very hot saw just after cutting will continue to increase in temperature for at least the first minute if you turn it off. Many a saw has seized while off due to this.
Also, with the bar tip embedded like this, its hard to get enough oil on the chain. This will also add friction/heat, and so will a dull chain, so keep it sharp!
Again, if you go this route, take it easy, but to be honest, I think that a 290 will still be a bit on the small side for such a big tree.