Ive bought a used civic 98, Ive noticed that if i put the car in D4 when the car is cold sometimes the car does not get in gear, this also sometimes happens with D3 when the car is going very slow and is cold sometimes the transmission downshifts and goes out of gear (like being in neutral), there are no problems with 2 or reverse

when the car is at normal operating temperature then there's no problem with D4 or D3. The shifting is smooth throughout. Question is: is this a serious problem ?? should I be concerned about it ? I think it might have to do with wear of the gears with age, mileage on odometer is 88000 km

the previous owner took very good care of the car in terms of maintenance , the engine is spotless and the interior is clean.

if anyone can recommend a good transmission fluid brand for use with this old civic to keep it in working condition, that would be a great help