Howdy! I switched at 50k to Mobil 1 and have 70k now.... did not experience any leaks. I say..down the hatch! ...and eyeball the engine here & there after the change to look for any leaking seals.
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'98 S70 T5

Synthetic oil can expose failing seals in higher mileage motors. Dino oil will deposit sludge around seals and actually help seal. Once you switch to syn, it cleans those areas and if a poor seal is present, leaks begin. As stated, the sooner the better to switch.

Synthetic oil has to be one of the best things you can do for your volvo!!!

I owned an 850 and now an S70 and use Mobil 1 Synthetic in both of them. Considering i bought both of them used I really wanted to make sure i was protecting my investments.........

The 850 "ate" oil in the beginning without ANY leaks at all. I must have had the volov dealer look at it atleast 10 times. Only thing i found out was that the motor goes through an "adjustment" period going from the regular to the synthetic motor oil, causing it to burn more oil.

Conversely, my S70 has never eaten any oil and has not sprung any leaks either.

I do (and so should anyone) check the motor oil level every 2000 or so miles. Considering, you can go 10K plus between oil changes on a synthetic oil i would suggest checking the oil level now and then.

Going through consumer reports, people etc etc i found Mobil 1 Synthetic to be the best synthetic oil in the market. Sure it does not help that the cheapest quart i ever found cost me $4 and i need 7 for my S70!!!!

Good decision on going with synthetic, you cannot go wrong......

I cannot understand why leaks would occur? I have been using synth oil since about the 50k mile mark and now have 74k miles with no problems.

Sahararon:

I changed to a full synthetic knowing there could be leaks and sure enough four months later our car had the rear main seal leak. The benefit of the protection from the synthetic and the eliminating of the lifter clicking out weighted the risk of of leaks. From reading this board the I gathered the rear main seal probably would have leaked at some time even without the switch. The rear main kind of was a blessing in disguise because the mechanic found the transmission leak we had and put a new PNP switch with no extra charge. If you are going to change to synthetic the earlier the better.

Robert - 94 855 NA 135,000 & 88 745T 220,000

If you don't have any leaks now, I would think it unlikely to cause a problem. I did have a leak and it became noticeably worse after the change.The benefits should outweigh the potential for a problem.

I converted to synthetic at 70K miles and haven't leaked a drop. Am now at 113K miles. Go for it!
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David
1998 S70 T5SE // misc mods (mostly lighting) // red calipers
1992 940 GLE // Hella Micro DE foglights and 170K