no one in his right mind would suggest 20w50 considering the hydraulic lifters of cultus.
old car is lame excuse people give
if seals are bad you better replace them than pouring thicker oil.
tvalve serves a blocker. when engine is cool (below optimal level) tvalve block the coolant from entering engine (water body) thus engine warms up fast to optimal temperature and as soon as it exceed that level, tvalve open the path for coolant.
early the engine warm up, the better. engine life decreases if tvalve is not present as it remain cool for longer period of time