I had my car serviced today. I asked the mechanic to inspect my engine for an oil leak, as I noticed a few drops of oil on the driveway a few weeks ago. They couldn't find one (although they gave the engine a degrease, so if it shows up again, it'll be obvious). For what it's worth, the rocker cover gasket and distributor cap seal have both been replaced.

They passed the remark that my car may be leaking some oil because of the viscosity of the oil I use. I use Mobil 1 5W-50, and have done so since I bought the car in 2006 with 71,000km - the car is now 11 years old and has 174,000km. It is serviced religiously.

They suggested that using a 15W-40 or 20W-50 oil instead may fix my leak. While I'm certainly not keen on going to a 20W-50 oil, but would there be merit in changing to something like Mobil 1 15W-50, or even Penrite HPR10 (10W-50) at the next service, should this oil leak remain unidentified?

Whatever oil leak it has, it isn't bad - I checked it just before the service, and the oil was still on full, and hadn't noticeably decreased since the previous service.

Thoughts about their suggestion?

Rubbish. If they cant find a leak, then why is there oil on the drive? A 5w-50 should be fine if its a healthy engine. A 15w-40 would be useful if the engine is tired and needs a higher viscosity during startup and cold running temps.

Are you sure its oil though and not coolant/water etc?

Thanks Dave. That's what I thought too.

It was up at my parent's place, my old man wiped it on his finger and smelt it and said it was definitely oil :(

I'll certainly be paying close attention to my garage floor in the coming days/weeks though.

Check the oil pan and any suspicious leaks/fresh oil down the sides of the block

I know this sounds weird but I've had this exact same mystery oil leak in the past on my TJ AWD and the thicker oil fixed it :|

What type/grade of oil did you switch to?

15W-40

Motor has done just over 230,000kms so yea, getting a little old.