[V8] Transmission won't go into 4th

[pdlord at talent.com.au]

I have a 1990 3.6 auto that seems reluctant to shift up to 4th gear. Was doing
this only when cold but now does so when the car has been driven for 20-30
minutes. Sometimes if I switch to manual mode it'll shift up. Sometimes it'll
do it in economy mode. It' intermittant.

At about the same time, with engine cold, driving away near home once, the
transmission didn't seem to be in any gear for a second as I tried to drive
away.

 I have recently flushed synthetic oil through twice and have a dipstick to
check level - which checks out OK. Could synthetic trans oil badly affect the
transmission?

I am going to flush the fluid again and replace the filter this time, but are
there any other suggestions?