If the transmission is slipping then you more than likely have a mechanical problem, not an oil problem. Typically transmission slippage is a result of one of two things. The first is low pump pressure, often caused by a sticking relief valve, or possibly even a dirty suction or pressure filter. Without enough pressure the clutch packs on each gear selection won't have enough pressure to engage fully and therefore will slip.
Then there are seals on the pistons that engage the clutches for each gear selection. If those seals are bad then the oil will bypass casuing low pressure and, once again, slippage due to inadequate pressure.
Lastly there is pure mechanical damage/wear in the clutches causing the slippage. Thing is this wear can be caused simply due to age and the amount of usage causing normal wear until the discs are worn out. Too this wear can be acelerated greatly by any of the causes I listed above because with not enough pressure to keep the discs tight together they will start to slip, and get worse and worse as material wears off during use.
For any of these problems, except the discs just being worn to the point that no amount of pressure will cause them to engage, as the oil gets hotter and thinner the pressure will tend to drop. Like I said before, as the pressure drops the clutches start to slip more and more.
Unfortunatly unless the problem is being caused by, and hasn't progressed past, something pump related (ie clogged filters, etc) than your only fix is a rebuilt/reseal on the transmission. Good luck.