I'm using 10W-60 Castrol Edge FST right now and my hydraulic tappets are a bit noisy on cold start. They rattle for like 2-5 minutes so I will change oils to thinner ones. My question is, would the popular ester based 10W-50 cure the problem, or should I go for even thinner oil like 10W-40 or 5W-50? Does 10W-40 still have good enough hot protection capabilities, as I'm not doing trackdays or very very hard driving on road, especially as 10W-40 was one of the oils that Ford Cosworth handbook proposes.

Car runs ~300 bhp, engine has done 100 000 miles, street used and I use it on summers mostly and ambient temperatures are usually +5-20 celsius.

I'd also like to hear your experiences on oils concerning the tappet noise that Cosworths are known for...