Would be a VERY good thing to change the oil. There isn't always a drain point though or you need a special tool as they are "designed" to be sealed for life. Be careful to find the correct fill point as some large allen key points may be securing mechanical gearbox parts. It is possible for gloupy horrible oil cause gear selection problems especially when cold/thick. Please change the oil anyway!

May not cure the problem though. Usually problems engaging first and reverse gears point to clutch drag of some sort.

A few thoughts:-

1) The new clutch may have been mis-fitted. May be worth bleeding the hydraulics ,( Is the biting point very low - near the floor?)

2) Is there an oil leak at the clutch end of the engine? Oil on the clutch plate can cause drag especially when first started.

3) You mention the list of the parts replaced in the past - have these been replaced in an attempt to fix this "long standing" fault?