My trans started slipping and I checked the.

My trans started slipping and I checked the fluid level and it was fine but I recently just read my car manual and found out the level should be checked while the car is running. Haven't checked it yet but wanted to know could a low trans oil level cause the slipping? My car now has 202,000 miles, I've been getting the oil changed at a Toyota dece I got it and tried to get my trans seause they said I shouldnt get it done cause it can damage it, been going to nifty lube the past couple of oil changes(plan to go back to Toyota) and they said I need a gasket and a filter, should Ijust check the level and see if its low and see if that fixes the problem or get it serviced? Sorry for the long question just freaking out cause this is my only car and its been running good and I've been taking care of it for the almost 3 years I've had it though I have put just over 50,000 miles in that time