Daihatsu YRV - sizes of gear oil and engine oil plug bolts

[YRV]

Hello again, already changed my coolant (pink/red G12+), gear oil (GL4+ 75w90), and engine oil (5w40 semi-synthetic).

I drive a lot and the next oil change is due soon, so I was wondering if I can change to a thicker oil..?

I was suprised to see TS-2 SHPD 20W-50 CG-4/SJ mineral (diesel truck oil) as the No. 1 OEM recommendation. Isn't this oil a bit too thick?

Optionally, the SCT Germany website (the producer of the cheapo MANNOL german oils I always use) lists substitutes such as 0w40 fully synth and 5w50 semi-synth. Would you agree with them? Wouldn't 5w50 or 20w50 be too thick? Or is _w50 ok for a relatively worn out engine such as mine?

Thanks in advance.