Peter, I lived in Zambia for quite a few years, and had the great pleasure of visiting Malawi many times. Where will you be when you visit? By May, the rains have pretty much ended. (May is a lovely time of year in southern Africa -- after the rains, so everything is green, and the sky is clear. It's warm and the evenings are fresh.) A couple of cautions, however: in late May, there is a cool and wet southeasterly wind which blows in from Mozambique, the ciperone. (Pronounced "chipper-oh-nee") It can bring cloudy drizzly weather which makes the laterite (dirt) roads slick and treacherous. It can bring heavy rain sometimes as well -- see Laurens van der Post's Venture to the Interior, an account of trekking on Mulanje in 1948, when the ciperone brought catastrophe. (I've written an account of a 6-day traverse, hiking across Mulanje, if you're interested -- send me a P/message if so, but don't feel obliged.)
Secondly, you might also encounter some wet weather if you're on the Nyika Plateau in the north, during May, depending on your dates. The rains can last into May in the north, at altitude.
Nothing to lose by using fenders and mudguards (mudflaps), as Dan's note signals. I would certainly use tires with a good dirt-road tread. Note that "gravel" as known in, say the US or Canada, or even Namibia, doesn't apply in Malawi: laterite is crumbly red soil with small bits of gravel, dusty in the dry and slick in the wet. The tarmac has potholes; avoid entering them, whether dry or full of water.
The coffee, BTW, is to die for. Go to the Mzuzu Plateau, home of Malawi's coffee. Immerse yourself in it, as it were. Then ride down a couple of thousand feet to the lake at Nkhata Bay -- more crowded than it used to be, but still beautiful. Careful about swimming in the lake, esp close to the shore if there are reeds and still water -- between the crocs and bilharzia, there are Things To Avoid. Ask advice from the people who live there.
Do check out the plateaux. I've mentioned Nyika, but the Zomba Plateau is fabulous too. (The "up" road will test your legs, the "down" road your brakes.) From the KuChawe inn on its SE corner, you have a splendid view across the valley, 80 kms to the NW rock face of Chambe, a peak on Mulanje, a mere 1600 metres and three days' climb, if that's the sort of thing you fancy. If not, just order a cold beer and look at it in the light of the setting sun, which turns the granite a bewitching golden rust.
And, Mulanje is a massif at 7500 feet, with 23 granite peaks on it (the highest summit, Sapitwa, is 10,000 feet), all of them scrambleable without having to resort to technical climbing. An old friend runs a trekking business on Mulanje, and I'm happy to refer you to him. My take on Mulanje, BTW, is: Knowing Mulanje, why would anyone bother with Kilimanjaro? Mulanje is more beautiful, more challenging despite being only half the height, and mercifully uncrowded. And, there are glorious streams and waterfalls. Happily, not many know about Mulanje, & that's why it's uncrowded. Also, the first day's ascent is very steep -- keeps away the dilettantes & riff-raff.
Time to stop, I'm getting weepy...
Good luck, and as you may guess, I'm hopelessly envious.
