I've been reading all about the viscosities of engine oil recommended by Volvo for use in the B230 engine. My 91 940GL has 244K kilometres on the clock and I use 15W-40 oil and have for about 2 years. I change the oil according to the service schedule (every 5-6 months or 8-10K kms but I'm now planning to reduce the time interval because of the mileage). I see that Volvo recommends this grade of oil only for demanding conditions. My driving is mainly highway at 100km/hr. The oil recommended for most conditions is 10W-30. My question is can I switch to 10W-30 after using 15w-40 for so long? It's coming into summer here and it does get very hot (often in the mid 30's celsius for days at a time). Would 10w-30 be okay for this sort of temperature? I am hearing a very slight solid knocking sound when the engine is cold. I'm assuming this could be piston slap or valve silencers. If it's piston slap is it possible using the thicker oil has caused this problem by increasing cold start wear? Would switching to the thinner oil reduce this problem? So many questions I'm sorry but I have become a bit concerned about this. I don't want to screw what is otherwise a good engine.
Thanks for any advice.

Brad