I was wondering, which one of the abovementioned solutions is the correct one. I am inclined to choose the latter, but I am open to your advice.
Kind regards
You're right. You are talking about quantity, so you want the plural ('one diesel', 'two diesels'). 'S is for possession ('my diesel's airfilter is clogged').

'Pair' is an alternate plural for 'pairs', so either is fine.
If you're talking about Diesel jeans, then it would be 'two pairs of Diesel jeans.'

'Two pairs of Diesels' is only acceptable in colloquial speech.
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They're pants, not engines, eh? If the subject is blue jeans, I think any formation would be colloquial speech.
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