White tractor blowing black smoke under strain

[sofasurfer]

White LT1850 lawn tractor. When straining in deep grass it starts blowing black exhaust. When not straining it does not smoke. It also runs just a tad rough and at high rpms it runs a little rougher. I see that that air filter is ready for replacement and the oil looks pretty dark. I will replace them tomorrow. Should I do the plugs also? What else may it be?
Thanks.

 

[cpurvis]

Sounds like it's running rich. Change the air filter and see if that fixes it.

 

[sofasurfer]

Thanks. Just praying its not the rings. Pray with me.

 

[cpurvis]

Oil smoke usually has a bluish tint to it and shows up at any speed.